Amazon (AMZN) launched a new service called Amazon Video on Demand, which allows users, for a fee, to watch any of 40,000 TV episodes or movies, in real time, on their Windows and Macintosh computers, and on specially equipped Sony Bravia TV sets. This service replaces an earlier Amazon video offering called Unbox.

I have been testing Amazon Video on Demand and I found it worked well, although it has some limitations. The user interface is clean and smart, the quality is good if you have a fast Internet connection, and there are some clever features. On the downside, it works poorly with the slow connections typical in places like hotels. And there are some studio-imposed limits on what content is offered and how you can view it.

To date, there have been three major models for legally getting TV shows and movies from the Web.

In one, best represented by Apple’s (AAPL) widely used iTunes store, the TV shows and movies are ad-free, but you pay for each. All of this content is downloaded to your Windows or Macintosh computer or your Apple TV set-top box for later viewing, even when you’re not connected to the Internet — though on Apple TV you can start watching while the material is being downloaded.

A second model, best represented by the studio-owned Hulu.com, presents movies and TV shows on a Windows or Macintosh personal computer free of charge, but requires you to watch commercials that can’t be skipped. The TV shows and movies you view on Hulu are “streamed” rather than downloaded, meaning they are meant to be viewed immediately, in real time, rather than stored for viewing when you’re not connected to the Internet.

The third online model is best represented by Netflix (NFLX), the popular DVD distributor. It offers a “Watch Instantly” streaming option on Windows PCs or on TVs equipped with a special set-top box, for a small portion of its large catalog of TV shows and movies. These videos are ad-free and don’t carry an individual charge, but require a monthly subscription fee.

The new Amazon Video on Demand service is a hybrid of these others. As on iTunes, the TV shows and movies it offers are ad-free and purchased individually, don’t require a subscription, and work on both Windows and the Mac, plus on one type of set-top box. In Amazon’s case, that’s Sony’s Bravia Internet Link, a $299 device that works only with Sony (SNE) TVs.

But, like Hulu and Netflix and unlike Amazon’s older Unbox service, the new Amazon Video on Demand service offers videos via real-time streaming. In many cases, it also allows downloading, iTunes-style, to Windows PCs (but not Macs) and to TiVo devices attached to a TV. The videos can’t be streamed in real time using a TiVo (TIVO).

Amazon’s streaming videos are viewed in any of the major Web browsers and don’t require any special software. I tested the new service on both Windows PCs and Macs, and on a Sony Bravia TV equipped with the $299 adapter box. In my tests, Amazon’s videos looked quite good over a fast Internet connection. However, on a typically lousy hotel Internet connection, the movies were often grainy and kept stuttering.

The new service doesn’t yet offer videos in high definition, something Apple just announced this week it is adding for some TV shows. Amazon says it is working on HD.

Amazon’s user interface for the new service is very nice. All of your purchased videos are available in a library stored on the company’s servers, so you can easily watch them again and again if they are purchased, or resume a partly watched rental. And the service remembers where you stopped watching a video and resumes it at that point, even if you started it on, say, a Dell (DELL), and resumed it on a Mac.

On the downside, the new Amazon service isn’t always simple or consistent. For instance, you can watch only two videos at a time, and not all titles can be either streamed and downloaded, or be either purchased or rented. Most rentals last 24 hours, but some differ.

Selection was OK, but not great. Because of studio policies, many current and recent movie hits aren’t available. There are gaps as well in the TV selection. For example, while iTunes offers the current second season of the excellent “Mad Men” series, Amazon has only the first season.

Prices also can be confusing. Amazon rents most movies for $3.99 and sells them for between $9.99 and $14.99. TV shows generally cost $1.99. But some titles carry different prices, albeit these are often lower.

All in all, Amazon Video on Demand is a good service for people who prefer paying for ad-free TV shows and movies, and is another strong step in the Internet’s rising competition with traditional TV.

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.

Now, two more set-top boxes have been launched to try to marry the Internet and the TV. Both adapters, from Sony (SNE) and Roku, worked well in my tests, but each has limitations. The problem is that one of the boxes supplies content from a wide range of Internet video sources, but only works on selected models of one brand of TV set; the other works on a wide variety of TVs, but only provides a single source of content.

Sony’s adapter is the Bravia Internet Video Link. This is a $300 module that attaches to certain Sony HDTV models. It can either be set up beside the TV or snapped onto the back of the set. Once it’s connected to your TV and to your home network for Internet access, a new menu appears on the TV allowing you a choice of videos from numerous online sources, including YouTube, Yahoo (YHOO), Blip. TV, Sports Illustrated, AOL, Wired, and the Web sites of CBS (CBS), Showtime and more.

Setting up the Bravia Internet Video Link was straightforward, even though it involved a welter of cables. There is no built-in Wi-Fi — you need either a cable or an add-on wireless adapter to connect to the Internet. The primary hookup to the TV is via a modern type of cable called HDMI, for High Definition Multimedia Interface.

I tested the Sony Link using the company’s most unusual HDTV set — a tiny, very costly model that uses a very thin, very vivid new screen technology called OLED, for Organic Light-Emitting Diode. This TV provided a spectacular picture, but it isn’t required to use the Sony module. The Link works on many larger, more common Sony sets with more common screens. It just doesn’t work on non-Sony TV sets.

Sony’s Bravia Internet Video Link adapter

The Sony module doesn’t have its own remote control. It uses the one that came with the TV. This makes for an awkward experience, because there are no standard play and pause buttons, and various other buttons on the remote meant to do one thing on the TV may do another when watching Internet video via the Link module.

Also, I found some of the Internet content to be disappointing. Many of the items labeled “movies” on various Internet channels were really just trailers, and some content was stale. For instance, some baseball news videos on Yahoo were weeks old.

However, Sony plans to make one of its feature films, “Hancock,” available through the Link module before it’s released on DVD. More important, it will be adding access to Amazon’s (AMZN) forthcoming video-streaming service, which promises to contain a wealth of full-length content.

The Netflix Player by Roku is much simpler. In fact, it was the simplest set-top box I have ever tested. It costs just $100 and does just one thing: It allows Netflix (NFLX) subscribers to view its movies and TV shows via the Internet on a television set instead of on a computer. It can’t get you any other video content from the Internet.

The Netflix player is a small, plain black box that works with most TVs, not just digital or high-definition models. It connects using both old-fashioned cables, like the kind used by many VCRs and older DVD players, or an HDMI cable.

Unlike the Sony, the Roku box includes both wireless and wired Internet connectivity, and has its own remote. While the box is capable of displaying high-definition content, the Netflix service isn’t currently streaming movies and TV shows in high definition, so you get varying quality, depending on your TV and Internet speed, up to DVD-type levels.

There’s no added monthly fee required to use the Roku box, but you must be a Netflix subscriber. The box merely displays the movies or TV shows you have placed in your Instant Queue on Netflix. You have to do this on your computer before viewing the videos on the Roku box. You can choose from around 12,000 streaming movies and TV shows, far fewer than the 100,000 titles Netflix makes available via DVD, but a sizable collection.

In my tests, the Roku box set up quickly and easily, the interface and remote were simple and effective, and the movies and TV shows I tested streamed quickly and without hesitation over my fast home Internet connection.

Both products are meant to promote sales of other things — Sony TVs and the Netflix movie-rental service. They do these tasks well, but neither is the breakthrough solution that will connect most TVs to most Internet video content.

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.

Beginning this summer, as soon as next month, wireless Internet access will arrive in the passenger cabins of some commercial U.S. airliners.

On these Internet-equipped planes, any passenger with a Wi-Fi enabled laptop — or a cellphone with Wi-Fi — will be able to do almost everything he or she could do online at home or at the office. That includes surfing the Web, using email, having instant-messenger text chats, downloading and uploading files, and streaming video and audio.

In fact, I did all these things a few days ago on a test flight using the new system, called Gogo. During the flight from San Francisco to Denver, on a small test jet, I could operate online as if I were sitting at my desk, or in a Starbucks. I used Dell (DELL) and Apple (AAPL) laptops, a BlackBerry (RIMM), a Windows Mobile phone and an iPhone to perform all the most common online tasks, while soaring over majestic mountains and glorious national parks.

I sent and received emails on Microsoft (MSFT) Outlook and Apple Mail, including messages with hefty attachments. I conducted IM chats on AOL (TWX) Instant Messenger and Google (GOOG) Talk. Using all the major Web browsers, I called up dozens of Web sites, and watched video clips on Hulu and YouTube. I downloaded photos, songs, PDF files and Microsoft Office documents. I used all the Internet functions on the iPhone, and on the Wi-Fi-equipped BlackBerry and Windows Mobile phone.

One important caveat: Gogo is a data-only system. It doesn’t allow phone calls and will block all services that allow voice conversations to be made over the Internet.

Gogo will launch on three American Airlines (AMR) routes, likely in July. The first planes to use it will be American’s 15 Boeing 767s flying between New York and Los Angeles, San Francisco and Miami. Later in the year, Gogo will be available on all of Virgin America’s small number of routes, and possibly additional American routes, if the first deployment works well. It’s supplied to the airlines by a Denver-based company called Aircell, which says it is in negotiations to offer the Gogo service on several other major U.S. airlines by next year.

The Gogo service will cost a flat fee of $12.95 for flights of three hours or longer, and $9.95 for shorter trips. You log into Gogo as you would any commercial Internet service, registering on a special Web page. Aircell plans to allow advance sign-up, so you’d only have to enter an ID and password on the plane. No add-on software, hardware or cables are required.

A few Web functions will be offered free from Gogo, including access to the American Airlines Web site, to Frommer’s online travel guides and to a limited selection of articles from The Wall Street Journal.

Gogo isn’t the first in-plane Internet service. A few years ago, Lufthansa (LHA.MU) offered a satellite-based service from Boeing (BA), mainly on over-ocean flights, but it was canceled.

The service operates at respectable, if not blazing, speeds — similar to what you’d get on a cellular broadband service or a slow home DSL line. On my test flight, download speeds varied from 266 kilobits per second to about 1.4 megabits per second, with the most typical speeds hovering between 500 and 600 kbps. Upload speeds were between 250 and 300 kbps. I found that most of the tasks I tested, except for streaming video, felt smooth and normal.

Speeds could degrade on a large plane with scores of people online simultaneously. But Aircell claims it has the technology to make my experience representative for anyone doing common tasks, such as Web surfing and email. During my test flight, eight laptops and six Wi-Fi-enabled smart phones were using the system simultaneously. All registered decent speeds, except for a couple of minutes when the plane was crossing between the zones controlled by the company’s ground-based towers.

Aircell gets Internet access to the planes through a network of 92 towers scattered across North America. These essentially are cellphone towers, carrying a high-speed cellphone data signal, except that the Aircell antennas point up, into the sky. A receiver on the underside of the aircraft picks up the signal, which is then distributed through the plane via Wi-Fi.

The companies say Gogo is safe and won’t interfere with the plane’s operation. It is government-approved, and pilots can shut the system off should they deem it necessary.

Gogo has some limitations. The service plans to allocate its capacity so that low-bandwidth activities like Web surfing and email take priority over high-bandwidth ones like streaming video. That means you may find video to be slow and halting.

And Gogo is a North American, land-based service only. It won’t work over the oceans and, for now, it won’t work on other continents.

But for U.S. travelers who want to stay connected in the air, Gogo does the job.

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

There are two models for this legal Internet distribution. Some shows and movies can be purchased or rented from services like Apple’s iTunes or Amazon’s Unbox. You pay a fee for these downloads, which don’t have commercials, and you can keep any videos you buy to watch repeatedly even without an Internet connection.

The other model, common on the Web sites of the TV networks, is free, ad-supported streaming directly within a Web browser. In this approach, you pay nothing, but you have to watch commercials that can’t be skipped. You must be connected to the Internet while watching, and you don’t get to keep the video.

This week, the ad-supported, streaming approach took a big leap forward with the launch of a Hollywood-backed service called Hulu, at hulu.com. Hulu aims to be a legal, one-stop shop for streaming of TV shows and movies from numerous networks and studios. It’s intended as an attractive antidote to pirate sites and to Google’s YouTube service, which has angered the media companies by allowing users to post all or parts of movies and TV shows without permission or payment.

Hulu is a joint effort of two big media conglomerates, NBC Universal and News Corp., each of which operates multiple networks and studios. (News Corp. also owns The Wall Street Journal and the Web sites where this column is published.) But Hulu contains programming from other companies as well, including Sony and Time Warner. All told, it offers full episodes or clips from about 400 TV series, plus 100 feature films.

I’ve been testing Hulu, and I am very impressed with its design and ease of use, and with the fact that it allows users to edit and re-publish its content on their own sites. Despite some drawbacks, it’s the first Web property I’ve seen from mainstream studios or networks that shows a real understanding of both modern Web design and the Internet’s culture of sharing. In my view, it’s far better than the typical network or studio Web site.

Even though Hulu lacks programming from ABC, CBS and many cable networks, it has a fair selection of popular shows, such as “30 Rock,” “The Office,” “The Simpsons,” “Battlestar Galactica” and “Saturday Night Live.” Its movie catalog includes old favorites like “The Usual Suspects,” “The Big Lebowski” and “Sideways.”

The site is organized in a clean, elegant manner. You can browse shows alphabetically, by genre or by network, or you can use an excellent search system. The search system even brings up links to videos of shows on other sites, such as ABC’s “Grey’s Anatomy,” that are missing from Hulu’s own collection.

Watching the material is a pleasure. You can view it in a fixed window or in full-screen view. You can also “pop out” the viewing window so you can place it anywhere on your screen and resize it to your liking. A feature called “lower lights” grays out everything on the computer screen but the video itself.

Even the advertising is relatively painless. TV shows contain just 25% of the commercial time that’s on regular TV. And Hulu allows you, in some cases, to choose the advertisers whose commercials you see, or else to opt to watch a movie trailer at the start of a video in exchange for seeing no further ads during that viewing.

In a break with Hollywood’s past rigidity, Hulu makes it easy to share, even edit, shows and clips. You can repost an entire video, or any portion of it, on your own blog or on social networking sites.

But Hulu also has some major downsides. Most important, Hulu lacks depth. Even with TV series from its owners’ own networks, Hulu typically contains only a small number of full-length episodes, and mainly offers short clips. In some cases, episodes expire after a while. For some shows, such as “Saturday Night Live,” there aren’t any full episodes, only clips. And the wildly popular “American Idol” isn’t in Hulu at all, even though it airs on News Corp.’s Fox network.

This stands in stark contrast to the depth offered on iTunes, where you can find multiple seasons of full episodes of many shows. And it doesn’t begin to compete with pirate sites, where you can find nearly everything.

Also, Hulu requires a decent broadband connection — a speed of at least 1 megabit per second is recommended, and even higher speeds are needed for some content. That means that using Hulu over the slowest DSL lines or cellphone modem cards will likely provide a poor experience.

Another problem is that, unlike iTunes or Amazon Unbox, Hulu can’t be used via a TV set-top box or a portable player. And shows can’t be saved for offline viewing, such as during flights.

Still, Hulu is a good start for Hollywood in finally providing a better experience for Internet streaming of TV and movies. If the service can add a lot more content and make viewing possible in more scenarios, it might strike a real blow against piracy.

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.