That is the question that Mozilla, the nonprofit organization that makes the leading alternative browser, hopes to answer this month when it releases version 3.0 of its Firefox Web browser. In some tech-industry circles, Firefox already is preferred over Microsoft’s (MSFT) Internet Explorer and Apple’s (AAPL) Safari, but it still isn’t used by most people, and Mozilla is hoping to broaden its appeal.

The new version will be released simultaneously for Windows and the Mac’s OS X operating system, as well as for Linux. While each of the three editions will have the visual style of the operating system on which it runs, all three will have the same features.

I’ve been using prerelease versions of Firefox 3.0 for months, and have recently been testing a near-final version and comparing it closely to IE and to Safari. I have tested it on multiple Windows PCs and Macs, on desktops and laptops, over slow connections and fast ones. I have tried it with well over 100 Web sites.

My verdict is that Firefox 3.0 is the best Web browser out there right now, and that it tops the current versions of both IE and Safari in features, speed and security. It is easy to install and easy to use, even for a mainstream, non-technical user. It can be downloaded, free, at mozilla.com by clicking on “Firefox 3 Sneak Peek.”

This situation may change. Microsoft is working on a new version of IE, scheduled to be unveiled later this year, with some impressive new features. And Apple is always working on new iterations of Safari, though it is secretive and hasn’t disclosed its plans. But for now, in my view, Firefox 3.0 rules on both Windows and Mac.

I couldn’t find any significant downsides to Firefox 3.0. Every page I tried rendered properly and rapidly on both platforms. I ran into only one glitch, in a preference setting. That problem appeared on only one of my four test machines and was fixable with the help of Mozilla, albeit via a geeky method.

In the one or two cases where Firefox lacked a feature I thought important, such as the “auto fill” feature in Safari that can quickly fill out an online form, I was able to find an add-on that did the trick from Mozilla’s vast library of add-ons, which are written by people all over the world. (One caution: Some existing add-ons won’t work with the new version until their authors update them.)

When Firefox first came out, it was the fastest browser, but it lost that title over the years. However, in my tests, this new third version of Firefox regained the speed crown. It beat IE 7 handily on my test Windows computers and edged Safari slightly on my test Macs.

For example, using a new Dell (DELL) XPS One desktop, I opened identical folders containing the same 16 bookmarks on both IE 7 and Firefox 3.0. IE took 37 seconds to completely display the 16 pages, but the new Firefox did it in just 23 seconds. On a new Apple iMac, I did a similar, but more daunting, test — opening identical folders containing 24 bookmarks. Safari rendered all of the pages in 36 seconds, but the new Firefox finished the job in 32 seconds.

The latest Firefox has a number of new and improved features. If you type any word or phrase into its address bar, the browser instantly searches your history and bookmarks for a possible match, to save you from typing or combing through your bookmark list.

The whole process of managing bookmarks has been vastly simplified. Every Web address is accompanied by a star icon at the right. To bookmark the site, you just click the star once. No other action is required. To specify where to file the bookmark, you click the star twice. You also can remove bookmarks by clicking the star. And you can tag bookmarks with key words, to make it easier to find them.

There are also smart bookmark folders, which gather your most visited sites, or most recently bookmarked sites, automatically into folders. You also now can more easily back up and restore your bookmarks, complete with tags.

Security is also improved. The old version of Firefox would warn you when a site you were visiting appeared to be a fake, designed to steal your identity. (IE has a similar feature, though Safari doesn’t.) But Firefox 3.0 now warns you about sites that are known for trying to plant viruses, spyware and other malicious software on your computer, a warning the other big browsers don’t yet provide.

With one click, Firefox 3.0 also provides details about who owns the site you’re visiting, and whether it’s encrypted, if the site owner has adopted a special type of security certificate.

My bottom line: Even though you already have a built-in browser, Firefox 3.0 can improve your Web experience.

• Find all of Walt Mossberg’s columns and videos online, free, at the All Things Digital Web site, walt.allthingsd.com. Email him at mossberg@wsj.com.

So, last month, the photo giant introduced Photoshop Express, its free Web-based service for storing, sharing and editing photos, in an effort to compete with established online photo services such as Yahoo’s (YHOO) Flickr, Google’s (GOOG) Picasa Web Albums, or the photo-laden Facebook social-networking service.

Photoshop Express has many of the same features as Flickr and its ilk. It gives you two free gigabytes of photo storage. But Adobe is hoping to make its mark with editing.

Most online photo services offer little or no editing, assuming you’ll do that using software on your computer before you upload your pictures. But Photoshop Express, borrowing from Adobe’s deep knowledge of photo editing, offers the nicest set of Web-based editing tools I have seen. They are sophisticated for a consumer application, yet easy to use. They edge out those in Picnik, a pioneering Web-based photo editor I hailed last year.

These slick editing tools are not only available for use with photos you’ve uploaded from your hard disk. You can also use them to edit pictures stored in your accounts at Facebook, Picasa Web Albums and another big photo-storage service, Photobucket — all without leaving Photoshop Express. You can even move pictures between Photoshop Express and these three services just by dragging and dropping.

Adobe’s new service is available in the U.S. only, at www.photoshop.com/express.

I’ve been testing Photoshop Express and its service overall is pretty good, even though it’s still labeled “beta.” It’s a nice example of the Web 2.0 trend, where programs accessed via a browser can look and feel like applications that live on your computer.

But Photoshop Express is rough around the edges. It can be slow at times, and it’s missing some obvious features, like the ability to easily download publicly shared pictures from other members or to print photos. Adobe says it is working on these things.

Photoshop Express isn’t meant to replicate all the features and power of Photoshop. It’s more like a Web-based version of Photoshop Elements, Adobe’s consumer software package.

I tested Photoshop Express on multiple computers: PCs running Microsoft’s (MSFT) Windows XP and Windows Vista, and Macs running Apple’s (AAPL) Leopard operating system. I used it in all three major Web browsers: Internet Explorer for Windows, Firefox on both Windows and Mac, and Safari, also on both platforms. It worked fine in all of these operating systems and browsers, though it does require Adobe’s free Flash software.

For my tests, I uploaded from my computers dozens of photos, from very large images captured with good digital cameras to smaller shots from cellphones. All were handled perfectly by Photoshop Express. I also opened and edited pictures in Photoshop Express from my accounts on Picasa Web Albums and Facebook. All of this worked well, though uploads of large images can be slow if your Web connection is pokey.

Photoshop Express is a handsome product, presenting your photos on a gray background with controls and features arrayed at the top and bottom, and down the sides, in a logical, clear manner. Your own photos are presented in a section called “My Photos,” and can be organized into albums. Photos that other Photoshop Express users have chosen to publicly share are organized into collections called “Galleries,” which can include multiple albums. You access these community photos by simply clicking on “Browse” or performing a search.

For each album you create, you can choose to share it publicly or to keep it private. Whichever option you choose, you can email friends either a link for viewing the album or a single photo. Your own photos can be downloaded at a variety of resolutions, including original size.

When you view shared galleries or albums, they appear as slide shows. You can select a number of slick effects by which the slides appear, allowing them to zoom and glide into place from various directions.

The editing features really stand out. In addition to standard tools such as auto-correction and red-eye elimination, Photoshop Express lets you touch up areas; adjust exposure, saturation, and lighting; and even make certain colors pop — so grass is greener, for instance. And, in most cases, it shows you small example images illustrating the changes, then previews those changes in the larger main image just by moving your mouse over the example. You can revert to your original at any time.

Unfortunately, there are a number of problems. Photos, especially large ones, can take awhile to appear in the editing module and to snap into focus. Captions sometimes get lost or mixed up when you move photos to other services. You can view shared albums only as slide shows, not as individual photos.

Still, Adobe has made a good start with Photoshop Express, and it’s worth a try if you want better online editing for your pictures.

Email me at mossberg@wsj.com. Find all my columns and videos online, free, at the new All Things Digital Web site, http://walt.allthingsd.com.

There are several reasons for this, including the security problems in the dominant Windows platform from Microsoft; spillover from Apple’s blistering success with its iPod music players; the fact that Macs can now run Windows programs; and Apple’s highly successful chain of company-owned retail stores.

But another key factor has been the Mac operating system, called OS X, which came out in 2001. It has proved to be as powerful and versatile for mainstream consumers as Windows, yet easier to use and more secure. And Apple has upgraded OS X far more rapidly than Microsoft Inc. has upgraded Windows, bringing out major new releases roughly every 18 months, while Microsoft struggled for more than five years to produce the latest Windows iteration, Vista, which came out in January.

On Friday evening, Apple will release yet another new version of OS X, called Leopard, to replace the current version, known as Tiger. I’ve been testing Leopard, and while it is an evolutionary, not a revolutionary, release, I believe it builds on Apple’s quality advantage over Windows. In my view, Leopard is better and faster than Vista, with a set of new features that make Macs even easier to use.

Leopard will come preinstalled on all new Macs. It can also be purchased for $129 as an upgrade to existing Macs that, depending on configuration, can be as many as six years old. Unlike Vista, which is sold in four noncorporate upgrade versions ranging from a $100 stripped-down “basic” edition to a $259 deluxe “ultimate” edition, there’s only one version of Leopard. It includes all the features, from those aimed at novices to those aimed at power users.

For me, the marquee features in Leopard are a new function called Time Machine that automatically backs up your entire computer in the background; two new methods, called Cover Flow and Quick Look, for rapidly viewing the contents of files without opening any programs; and new techniques that allow you to access the files in, and to remotely control, other computers on your network or connected over the Internet with a few clicks and no technical expertise.

Plus, Apple’s free software for running Windows on a Mac, called Boot Camp, which was formerly an add-on users had to download and install, is now built right into the operating system. And, in my tests, the third-party Fusion program for running Windows and Mac programs simultaneously continued to work fine in Leopard.

I did notice a few drawbacks, but they were minor. The menu bar is now translucent, which can make it hard to see the items it contains if your desktop picture has dark areas at the top. The new folder icons are dull and flat and less attractive than Vista’s or their predecessors on the Mac. While Time Machine can perform backups over a network, the backup destination can only be a hard disk connected to a Mac running Leopard. And, on the Web, I ran into one site where the fonts on part of the page were illegible, a problem Apple says is known and rare and that I expect it will fix.

While Apple claims the new system includes more than 300 new features, there is nothing on the list that could be considered startling or a major breakthrough. Some of Leopard’s features are unique, but many others — such as backing up data and quickly viewing files — have been available on both Windows and the Mac via third-party programs or hard-to-find geeky methods buried in the operating systems. Leopard has made them easy to find and use.

When I upgraded my personal iMac desktop to Leopard, it took less than an hour, and after the process was complete, all my programs, including the Mac version of Microsoft Office, the Firefox Web browser and Adobe Reader, worked rapidly and fine. I was still able to run Windows XP via Fusion. And my previous installation of Boot Camp, which turns the iMac into a speedy, full-fledged Vista machine after a reboot, worked perfectly. All my Vista programs and files continued to function properly.

With Cover Flow, users get a visual preview of a computer’s files without having to open programs.

In fact, every piece of software and hardware I tried on two Leopard-equipped Macs — a loaned laptop from Apple and my own upgraded iMac — worked fine, exhibiting none of the compatibility problems that continue to plague Vista. My old Hewlett-Packard inkjet printer, for which Vista lacks the proper software, worked instantly in Leopard, even over the network. And, unlike with Vista, it was able to print on both sides of the page. I popped my old Verizon cellphone modem card into the test Leopard laptop and it worked, too, with no software installation or tweaking.

Leopard felt about as fast as Tiger, and it started up much faster than Vista in my tests. I compared a MacBook Pro laptop with Leopard preinstalled to a Sony Vaio laptop with Vista preinstalled. Even though I had cleared out all of the useless trial software Sony had placed on the Vaio, it still started up painfully slowly compared with the Leopard laptop.

It took the Vista machine nearly two minutes to perform a cold start and be ready to run, including connecting to my wireless network. The Leopard laptop was up, running and connected to the network in 38 seconds. In a test of restarting the two laptops after they had been running an email program, a Web browser and a word processor, the Sony with Vista took three minutes and 29 seconds, while the Apple running Leopard took one minute and five seconds.

Here’s a rundown of some of Leopard’s key features. Much more detailed information is available at apple.com/macosx.

File management: Apple’s Finder, the equivalent of Explorer in Windows, now offers two new ways to quickly see what your files contain. You can still view them as icons or lists. But you can also use Cover Flow, the same system Apple uses in iTunes and on the iPhone to display album covers for music. In Leopard, a large preview of each file you select appears above the list of files in a folder, and you can rapidly scroll through these icons. These previews are live, and their contents can be viewed without opening the program that is normally needed to display them.

Time Machine backs up files.

For instance, if the file is a video, you can just click on it, and it will play. If it’s a multipage PDF file, you can click on it, and arrows will appear allowing you to flip through the pages.

An even better and deeper look can be obtained using a feature called Quick Look. Just hit the space bar or click on a toolbar icon, and a preview of any selected file zooms out. You can even view multiple sheets in an Excel file via Quick Look without launching Excel.

Another quick new way to see your files is available in the Dock, the Mac’s equivalent of the Windows Task Bar. Here, any folder you place on the right side of the dock will display its contents, after a single click, either as a grid of icons displaying miniversions of the file or as a “fan,” or arc, of such icons. These special Dock folders are called “Stacks.” Leopard includes one by default that is the destination for everything you download from the Internet, so your desktop will no longer get cluttered with downloads,

Time Machine: This built-in feature will continuously back up all of the contents of your Mac to either an external hard drive directly connected to the computer, or to a hard disk connected to another Mac running Leopard that’s on your network. The initial backup, in my tests, took all night, but after that, the system updates the backups hourly and I didn’t notice any slowdown during the process.

To recover any file you deleted, you simply click on the Time Machine icon, and you are taken to a view that shows file folders — or your email or address book or photo collection — in a stack of windows that appear to go on infinitely. You click on an arrow and the stack of windows zooms until you arrive at the last view in which the missing file existed. Then, you click “restore,” and the file is recovered in your normal desktop view. You can also restore whole folders, groups of files, or even an entire hard disk.

Shared computers: In Leopard, any computer that has been set to be shared on your network shows up on the left side of every Finder window. Click on it, and you can access whatever folders have been shared on those machines. Depending on the remote computer’s security settings, you may first have to enter a user name and password. It’s the simplest method I’ve ever seen for accessing other computers on a network. And it works with Windows PCs as well as Macs. When I first turned on the Leopard laptop in my office, it immediately found a shared folder on my colleague’s old Dell running Windows XP. She hadn’t even remembered sharing the folder, which contained files from 2003.

You can copy or move files to and from these shared computers, or view their contents with Cover Flow and Quick Look, or open them in programs on your own computer.

If you are a member of Apple’s optional .Mac service, which costs $100 a year, you can use a feature called “Back to My Mac,” which can access your Macs from thousands of miles away over the Internet. However, this feature works only over certain kinds of routers (not all of them Apple’s) and, as my router didn’t qualify, I couldn’t test it.

Remote control: For any Mac in your shared-computers list for which you have permission, you can take over the screen by simply clicking on a button called “Share Screen.” You can also remotely control distant Macs over the Internet using Apple’s built-in iChat instant messaging program, as long as you have permission and the Macs are running Leopard.

Stacks displays the files in folders in the dock.

iChat: Apple now allows you to use its instant messaging program with Google Talk as well as AOL’s AIM service, and you can set up a video chat in which you can present a slide show or display a document. You can also add special backgrounds that can make it look as though you’re someplace else, like Paris. In my tests, this even worked with someone on the other end using a Windows XP computer running the latest version of AIM.

Spaces: In order to cut down desktop clutter, Leopard lets you set up as many as 16 different desktops that can run simultaneously, with different programs open in each. You switch among these desktops by using keyboard commands or a menu.

For instance, you might have your iPhoto and iTunes running in one “space,” or desktop, your Web browser and email program in another, and Windows XP in another.

Leopard isn’t a must-have for current Mac owners, but it adds a lot of value. For new Mac buyers, it makes switching even more attractive.

Email me at mossberg@wsj.com. Find all my columns and videos online free at the new All Things Digital web site, http://walt.allthingsd.com.

The result is a polished, fairly powerful email program that I prefer to Google’s much-hyped Gmail, which is undergoing an even longer gestation. It has been in beta status since April 2004.

I’ve been testing the new Yahoo Mail on both Windows and Macintosh computers. It has some downsides, but it beats Gmail, in my view, both in terms of features and in terms of its ability to act like a standard computer program rather than a Web page, something for which Gmail often gets more credit.

A closer competitor to Yahoo Mail is actually Microsoft’s Hotmail, now called Windows Live Hotmail. But Yahoo tops Hotmail, too, in my opinion.

The new Yahoo Mail, which works in Internet Explorer and Firefox on Windows, and in Firefox on the Macintosh, is now more than just an email program. Like Gmail, but unlike Hotmail, it has a built-in instant-messaging module. You can choose to communicate with any of your contacts via a real-time chat, right from within Yahoo Mail, as long as that contact is online and has an IM account on either the Yahoo or Microsoft instant-messaging networks. You don’t need to be running your IM program.

Unlike either of its competitors, however, the new Yahoo Mail also allows you to exchange text messages with people on cellphones, although the message exchange must be initiated from Yahoo Mail.

Yahoo Mail offers unlimited storage of emails and attachments free of charge, and a very fast and good search capability — like Gmail’s — so you can keep years of messages on hand and retrieve them quickly. Gmail offers 2.9 gigabytes of storage free. It sells extra storage for prices ranging from $20 a year for six gigabytes to $500 a year for 250 gigabytes. Hotmail is in the process of boosting its storage to five gigabytes, free, and 10 gigabytes for $20 a year.

With Yahoo Mail, you can send attachments of up to 10 megabytes per message and 20 megabytes if you opt for a $20-a-year plan that also eliminates the annoying banner ads that litter the free version. Gmail offers attachments of up to 20 megabytes free. Hotmail allows 10 megabyte attachments and 20 megabytes under its $20-a-year plan, which also banishes ads. Gmail has no banner ads, just text ads that run alongside the emails and can’t be eliminated.

This new Yahoo Mail is gradually being rolled out in coming weeks. The company still plans to retain the older version of Yahoo Mail, now called Classic, for people who prefer it, or for those using browsers that are incompatible with the new version, such as Apple’s Safari.

The new Yahoo Mail allows you to do things that once were impossible in a Web-based email program. For instance, you can drag messages to new folders, or select a group of messages in the same way you would with a standard email program, to delete them or mark them as read or unread. Unlike in Gmail, when you right-click on a message you get a list of options that pertain to the mail program — like “Reply to Sender” — instead of options that pertain to the use of the browser — like “Add to Favorites.”

Also, Yahoo Mail features a very nice tabbed interface that Gmail and Hotmail lack. With this interface, which is separate from the browser’s own tabs, you could have your inbox in one tab, an instant-message or text-message conversation going on in another and a new email you are composing occupying yet another. You can move among these tabs without losing the content in any of them.

And like Hotmail but not Gmail, Yahoo Mail offers a preview pane, like Microsoft’s Outlook, so you can see the contents of an email without opening it. Gmail offers just a “snippet” of the message content. Unlike Gmail, which forces you to view your emails as bunched-up “conversations,” Yahoo Mail — like Hotmail — displays them as a standard email program does, sorting them by date, sender, subject or size.

So what are the downsides of Yahoo Mail?

Well, the biggest is probably that unless you pay for the $20-a-year premium plan, you can’t view your Yahoo mail account in a standard email program such as Outlook or Apple Mail. Gmail allows this free of charge. Hotmail allows it free in Outlook and in Windows Mail, though it will soon announce the capability for other email programs for premium members and, eventually, for free members as well.

Also, I found Yahoo Mail could sometimes be slow. When I created a new folder and tried to drag 200 emails into it, I was warned that I couldn’t do that because the new folder was “still being created.”

But overall, Yahoo did a really good job making its online mail program versatile, powerful and accessible.

Email me at mossberg@wsj.com. Find all my columns and videos online free at the new All Things Digital Web site, http://walt.allthingsd.com.

Dell has been having a tough time lately. Once the widely admired leader among PC makers, it has lost its sales crown to Hewlett-Packard and has seen its once-legendary customer satisfaction plunge.

In an attempt to jazz up its image, the stodgy behemoth recently introduced a line of thin, sleek, colorful consumer notebooks, the XPS M1330 series, but has been unable to build the machines on schedule.

Now, Dell is making another interesting move to better address the market. It has introduced a new sub-brand of computers intended for perhaps the most poorly served segment of computer buyers: really small businesses with 25 employees or less that have no full-time IT departments.

The new Dell line for these very small businesses is called Vostro. It fits into a new Dell brand lineup. All Dell’s consumer models, both laptop and desktop, are called Inspirons, a name formerly reserved only for laptops. Big business models are still called Latitude, Optiplex or Precison. High-performance consumer models are called XPS.

I’ve been testing a midrange Vostro model, a laptop with a 15.4-inch screen called the Vostro 1500. It’s a bulky, plain machine with nothing built in that Dell doesn’t already offer on consumer models, and some omissions that could make for extra setup work for small businesses. But it is fairly affordable and, to Dell’s credit, it comes without the useless trial software Dell and others cram onto consumer PCs.

Judged by its hardware and software, the Vostro 1500 is mostly a branding-and-marketing ploy at the moment. Dell concedes the new line is “just a first step” in what it swears will be a major initiative to serve very small businesses.

The only real small-business innovation lies outside the product itself: a dedicated phone support staff that Dell says has been trained specifically to serve these small companies. But unless you pay extra, you’ll still be dealing with support people based outside the U.S. — in Canada and in the Philippines, far from Dell’s Texas headquarters. Only time will tell if these folks can really provide better-than-usual service.

The Vostro 1500 starts at just $549, but that base model has very little memory and a small hard disk. The configuration Dell lent me for testing, with a decent two gigabytes of memory and a 160-gigabyte hard disk, plus an extra-large battery and a built-in Sprint cellular modem, costs $1,701. Dell says it’s available today at a promotional price of $1,327. The optional Sprint service requires a monthly fee, usually $60.

There are three other Vostro laptop models, including a very basic design called the Vostro 1000 that starts at just $449. Dell also offers Vostro desktops.

The Vostro 1500 I tested came with Windows XP instead of Windows Vista. Most Vostro models can be ordered with XP, which may be a welcome choice for small businesses that don’t want to wrestle with the continuing lack of compatible software and hardware for Vista.

Dell touts several features that it builds into the Vostro machines. One is a network setup program called Network Assistant. Another is PC Tuneup, which is maintenance software. A third is an online backup service that stores up to 10 gigabytes of data.

These are all useful, but none is new or tailored to small businesses. All were already available for Dell’s consumer models, albeit for small fees. Even the choice of Windows XP and the option to order a computer without any trial software have been available on some Dell consumer models.

Still, there were some surprising omissions on the Vostro that I tested. There was no security software, not even the usual 90-day subscription. Dell notes that you can opt to get the 90-day subscription free of charge and buy a 15-month security package for $79. It figures that many small businesses might already have access to security software. But because this is a necessity, installing it after the fact could be a hassle.

The Vostros with XP, like my test unit, also lack a modern, secure Web browser. They come with the old, insecure Internet Explorer 6.0 instead of Mozilla’s Firefox or Microsoft’s much safer Internet Explorer 7.0. A small business buying a Vostro with XP would have to immediately replace the browser.

And the Vostro doesn’t come bundled with any small-business software.

In my tests, the Vostro 1500 performed just fine, running quickly and easily connecting to my Wi-Fi network. But it’s a heavy, thick box that would be a burden for travelers. My configuration tipped the scales at seven pounds.

Yet, the battery life was only fair for such a bulky laptop, even with the extra-large battery. In my harsh battery test, where I turn off all power-saving software and play a loop of music, the Vostro 1500 lasted three hours and 36 minutes, meaning you’d likely get five hours, at most, in normal use. The standard battery would get you about a third less power.

Overall, the Vostro 1500 is just an OK laptop, which isn’t different enough from consumer models to really earn the small-business designation it claims.

Email me at mossberg@wsj.com. Find all my columns and videos online free at the new All Things Digital web site, http://walt.allthingsd.com.

Corrections & Amplifications:

Dell Inc. has introduced a computer line named Vostro for users at very small businesses. An earlier version of a headline on this column incorrectly said the line was named Vostra.

In classrooms and offices, homes and coffee shops, Macs are far more visible in the U.S. than they were just a few years ago. Part of this success results from the fact that Macs are excellent machines that handle the most important and common tasks as well as — or better than — computers running Microsoft Windows.

But the new popularity of the Mac is also partly due to the fact that it can now run Windows along with Apple’s superior Mac OS X operating system. That means that if there’s a program you need that comes only in a Windows version, you can run it on any current Mac model, speedily and with all its features.

Starting next week, there will be a new way to do this. A company called VMWare, long the leader in what’s called “virtualization” — running multiple operating systems simultaneously on a single computer — will be selling a program called Fusion that allows Windows, and Windows programs, to run on a Mac.

I’ve been testing Fusion, and I’ve found it works well. For instance, as I write this column on a Mac laptop using the Mac version of Microsoft Word, Fusion is allowing me to simultaneously run several popular Windows programs — Microsoft Outlook, Windows Media Player and Internet Explorer. Each is running in its own window, just as if it were a native Mac program. I can switch from one to the other rapidly and smoothly. Their icons show up on the Mac’s “Dock,” just like the icons for Mac programs.

In my tests, Fusion never slowed down my MacBook Pro laptop or two other Macs on which I tested it. As I write this, the Mac version of Word and all three Windows programs are performing normally, even though I am also running five other Mac programs. My MacBook Pro, which isn’t the latest or most powerful model, has two gigabytes of memory and an Intel processor that is a generation behind the current model.

Fusion, which will be available for $80 at vmware.com, becomes the third major option for running Windows software on a Mac. It will go up against a fine program called Parallels Desktop, also available for $80 at parallels.com and at retail stores. The third option is Apple’s own Boot Camp, currently a free product available at apple.com/bootcamp. Boot Camp will become a built-in feature of the next version of Apple’s Mac OS X operating system, due in October.

All three programs require users to purchase a full version of Windows and install it on the Mac. Like Fusion, Parallels is a virtualization program that allows you to run Windows and Windows programs simultaneously with the Mac operating system and Mac programs. Boot Camp works differently: It requires that you restart the Mac to switch into Windows, and it runs only one of the operating systems at a time.

In my tests, I compared Fusion and Parallels, which is its closest competitor. I used Windows XP Professional. Each also works with the new Windows Vista (and with older versions of Windows and various versions of the Linux operating system). But Microsoft has imposed a legal prohibition on installing the most common consumer versions of Vista, Home Basic and Home Premium, via virtualization programs.

The two programs are very similar. In most scenarios, they function nearly identically. Both allow you to run the full Windows desktop either in a window on your Mac or in full-screen mode. Alternatively, both allow Windows programs to float on their own, with the Windows desktop hidden, so they look and feel just like Mac programs. Both permit you to fetch and save files from folders already on your Mac. Both support copying and pasting between Mac and Windows programs. Both automatically use your Mac’s Internet connection.

Parallels has more features than Fusion. It comes with a set of utilities Fusion lacks, such as a program that can migrate the contents of a physical Windows PC into a Parallels virtual Windows PC, and another that allows you to retrieve files from the virtual Windows machine even when Parallels isn’t running. Parallels also has a nice feature that lets you assign any file to automatically open in a Windows program instead of a Mac program. And it makes it much easier to use a printer over a network than Fusion does.

But I found Fusion puts less strain on the computer overall. While I like Parallels and have used it since it came out, it sometimes slows down my Mac, especially when it is starting up Windows or performing some other major task. Fusion has a much smaller impact on the Mac’s overall performance.

You won’t go wrong with either program. Both give the Macintosh a level of versatility that can’t be matched by Windows-only machines.

• Email me at mossberg@wsj.com. Find all my columns and videos online free at the new All Things Digital Web site, http://walt.allthingsd.com.

Such Web-based software has existed for years, but it was clumsy, slow and simplistic — no match for locally installed software. Common techniques, such as dragging items around the screen, were impossible. Seeing the results of an action often required the Web page to reload.

Now, developers are churning out Web-based applications that are so fast, rich and smooth they can hardly be distinguished from standard programs. And because they live online, these Web applications can be constantly updated; can run on both Windows and Mac computers; and can be easily integrated with other Web sites and services.

One of the best examples of these slick new Web-based application is Picnik, a sophisticated, photo-editing application offered free of charge at picnik.com. I have been testing Picnik and I like it a lot. It’s a fast and impressive program for tweaking and improving your photos, then posting them to popular photo Web sites, saving them to your own computer, emailing them, or even printing them.

Picnik, which comes from a small Seattle company called Bitnik, isn’t meant to compete with Adobe Photoshop, or to serve professional photographers or dedicated hobbyists. Instead, it’s for the same casual photographer who would use the limited editing tools in Apple’s iPhoto or Microsoft’s Windows Vista Photo Gallery.

Picnik isn’t a place to store your pictures, or a way to organize them — yet. The company says it will consider adding these features down the road. For now, it is focused on being an editing complement to popular Web services — such as Yahoo’s Flickr, Google’s Picasa Web Albums, and the independent Facebook — that already allow for storing and organizing photos. You could also easily use it as the main editor for photos you store on your hard disk.

The program is currently in beta, or test, phase, though in my tests it worked smoothly and surely. During this beta period, all of its features are offered for free. Later this summer, the company expects to end the beta period and begin charging something like $20 or $25 a year for access to some of the more rarified special effects that Picnik offers, though the core editing and sharing functions, and some of the effects, will remain free.

In my view, Picnik has a beautiful and responsive user interface that worked perfectly on the multiple Windows and Macintosh computers I used to test it. It worked equally well in the latest versions of the three best-known Web browsers: Microsoft’s Windows-only Internet Explorer, Mozilla’s Firefox (on both Windows and Mac) and Apple’s Safari (on both Mac and Windows.)

Picnik uses a simple tabbed interface across the top to navigate among its major functions. Edits and changes are previewed in real time, instantly, without the need for a page refresh or reload. Actions are confirmed with translucent messages that pop up on the screen and fade gracefully.

Any edit or special effect can be undone or redone instantly, all the way back to the original version of the picture, which Picnik retains on its servers during the editing process.

For example, you can zoom in or zoom out on a picture with a slider that works just as it would in a local program — the effect is immediate, with no jerkiness. If you wish to crop a picture, a pane representing the region to be included in the crop is superimposed on the photo. Everything inside the pane is sharp and clear, and everything else is faded a bit. This pane can be dragged, or resized, in real time.

Another example: If you want to tint a picture, the program shows you a color palette with a white dot you can move around the palette to pick your tint. As you do this, or move a slider that controls the intensity of the tint, the changes are instantly previewed in the picture.

None of this is unusual for a standard photo program installed on your computer, but it is impressive to see these effects happen so quickly and interactively in a program functioning over an Internet connection.

Picnik’s makers have struck partnerships with Flickr, Picasa and Facebook, and you can easily fetch pictures from these sites and post new pictures or edited versions of the originals back to the sites. You don’t need to switch to the sites themselves, they appear inside the Picnik Web page.

You can also upload pictures for editing from any other Web site, or from your hard disk, and you can email pictures to friends or to a wide variety of other sites, such as PhotoBucket, SmugMug and Snapfish.

The designers of Picnik have done such an elegant job that I wish the site would allow storage of photos, or organization of photos across your multiple online accounts and your hard disk. If you want to see how good a Web application can be, take Picnik for a spin.

• Email me at mossberg@wsj.com. Find all my columns and videos online free at the new All Things Digital Web site, http://walt.allthingsd.com.