(FORTUNE Magazine) – "Extraordinary how potent cheap music is," Noel Coward wrote. Sure enough, the 99-cent legal song download is having a potent effect on the music industry as we near the first anniversary of the Apple iTunes Music Store. Apple says it has sold more than 30 million songs at 99 cents per. Although billions of songs still trade hands illegally each month on file-sharing services like Kazaa, analysts expect that within five years online music will provide as much as a third of the music industry's sales, worth several billion dollars.

That explains why more than a dozen services have sprung up to sell music to consumers over the Internet. Besides Apple's store, which controls some 70% of all single-song downloads, there are BuyMusic, eMusic, MusicMatch, MusicNow, RealPlayer, Rhapsody, Napster, and even a service from Wal-Mart. Computer companies, online service providers, and consumer electronics stores are joining the chorus; in coming months we'll also see online music services from big-name headliners Microsoft, Amazon, and Sony.

In these pages we've considered only Windows-based download services. All but two of the services listed in the table sell songs in Windows Media Audio (WMA) format, which the Apple iPod can't stomach but which plays well on dozens of other devices. Subscription-only services eMusic and Rhapsody don't appear in the table but are discussed at the end of this article.

iTUNES MUSIC STORE If you're new to online music, this is the place to start. It's based on the Windows version of Apple's iTunes software, which tightly integrates a media-player program (iTunes), a download service (the music store), and a portable player (the iPod), resulting in unparalleled ease of use. The software is free and there are no subscription fees. The catalog of more than 500,000 songs is among the largest of all legal services. Apple's search functions are best-in-class, making it easy to find the music you seek, assuming it's available. Any song you buy can be burned to a CD an unlimited number of times; unchanged playlists can be burned to a disc up to ten times. You can transfer songs for playback on as many as three computers, and to your iPod as many times as you want.

Nice features include celebrity playlists (what's Sting listening to now?), iTunes Essentials (the best make-out songs, for example), Billboard charts from several decades, prepaid cards and allowance accounts, gift certificates, and--rejoice!--a large selection of audio books. On the downbeat, downloaded iTunes songs use a proprietary-rights management system called FairPlay that's not yet supported by other portable players, meaning you're locked into using an iPod (there are worse fates). If you already have a large collection of songs in the Windows Media format, you're better off somewhere else; iTunes will, however, work with your MP3 files.

BOTTOM LINE: This is the most popular legal download service, and with good reason.

BUYMUSIC BuyMusic claims to carry 400,000 songs, but its catalog still seems more limited than most. It was one of the first services I tried, and one of the first I abandoned. Pricing is inconsistent, though bargain hunters can occasionally find tunes for 79 cents. Worse, digital-rights management is hopelessly confusing, varying song by song. How many times can you burn a song to a CD? Can you listen to it on more than one computer? Can you transfer it to a portable player like the Creative Nomad? You don't know until you try to buy the song. I buy music to relax, not to worry about keeping track of legal restrictions.

BOTTOM LINE: Bargain hunters only.

MUSICMATCH DOWNLOADS MusicMatch Jukebox is an excellent Windows music-management program, and the download service makes it even better. More than 400,000 songs are offered, and audio quality is above average. This hybrid service offers both downloads (you own the music) and, for $2.95 or $4.95 a month, depending on features, a streaming service (you listen to as much music as you want but don't own it and can't burn it). Downbeat: When I selected the New Age genre and clicked the subgenre Nature Sounds, MusicMatch suggested I might like "I Don't Give a Fuck" by the hip-hop artist Jermaine "Nature" Baxter.

BOTTOM LINE: If you use MusicMatch Jukebox already, give it a spin.

MUSICNOW Another hybrid service, MusicNow offers downloads of more than 400,000 songs. The optional All MusicNow service ($9.95 a month) provides unlimited streaming of commercial-free music channels, great for exploring new music. The basic service, however, doesn't really excel at anything in particular.

BOTTOM LINE: MusicWhatever, unless you pay extra for the premium service.

REALPLAYER 10 MUSIC STORE This service requires proprietary software, the RealPlayer 10 media player, but the payoff is the best audio quality of any service. The download library has about 500,000 songs. Like iTunes but unlike the other services, Real supports the AAC compression format, which means songs play on a limited range of portables (including some Palm handhelds and the Treo 600).

BOTTOM LINE: If you care enough to hear the very best.

NAPSTER Another hybrid service, offering a wide selection of downloads plus an optional $10 monthly premium service with access to message boards where users can swap ideas, listen to expertly programmed radio channels (including ones you can program yourself), and see other users' favorite playlists. The premium service permits unlimited renting of songs on your computer, but they can't be burned unless you buy them and can't be played if the subscription lapses. Napster is among the most innovative services, and the music information and expert recommendations are excellent.

BOTTOM LINE: The premium service is great for exploring new tunes, assuming your budget can handle $120 a year on top of the cost of purchasing tracks.

WAL-MART MUSIC DOWNLOADS This is a "beta" version; Wal-Mart plans to launch the full service this spring with a greatly expanded music library and added features. Until then, though, it'll take your 88 cents for any of a limited selection of songs. Come here if you're willing to give up frills to save 11 cents a song. Wal-Mart offers only "clean" and "ultra clean" versions of songs with explicit lyrics. Even so, you can download the tender ballad "F**k It (I Don't Want You Back)" by rapper Eamon.

BOTTOM LINE: Will Wal-Mart undercut the competition? Stay tuned.

eMUSIC For music lovers whose tastes are decidedly not mainstream, eMusic offers more than 250,000 songs from independent artists, and nothing from the major labels. Subscriptions start at $10 a month, with a fixed number of downloads, depending on the plan. Choose wisely, and the price can be as little as 20 cents a track. Songs are unprotected MP3s, encoded at a variable-bit rate for highest quality, meaning that you can do with them what you wish. Human experts in various genres, as opposed to pattern-matching robots, offer helpful recommendations.

BOTTOM LINE: Superb for indie fans. Check out the free trial at eMusic.com.

REALRHAPSODY This is the premier subscription service. For the price of one CD a month, you can listen to full-length songs and CDs as much as you want in the largest music store on the web and receive dozens of ad-free radio channels. For music exploration, it can't be beat. If you hear a song you like, buy it for 79 cents. If you spend all day at your computer and have an always-on broadband connection, it's rhapsodic. But unless you pay extra, the music stays on Real's computers, not yours.

BOTTOM LINE: Expensive, but there's a 14-day free trial at www.real.com/rhapsody.

FEEDBACK technology@fortunemail.com