NEW YORK (CNN/Money) - Are users ready to pay for online music services when they can still rip, mix, and burn?

Music consumers have been spoiled by free, file-sharing sites, such as KaZaA and Morpheus, which allow them to download any number of songs on demand, much to the ire of the music industry, which claims this has hurt record sales.

Several subscription services now exist that hope to lure customers by offering what they call a respectable catalog of songs, improved features for users, plus reviews, musicians' bios and more.

"This is a market that hasn't fully developed yet," said Alan McGlade, CEO of MusicNet, one of the subscription services. But the business is gaining "momentum," he said, with the launch of MusicNet on AOL and more recently, iTunes from Apple Computer.

The research firm Gartner predicts that online music services will generate about $660 million in revenue by 2006, up from an estimated $89 million this year. The firm also estimates that the number of subscribers will leap more than tenfold to 5.8 million in 2006 from about 500,000 at present.

The number of legitimate online music services is growing, albeit slowly. MusicUnited, sponsored by the Recording Industry Association of America, a group of record labels, has a comprehensive list of legitimate online music sites.

If you aren't sure where to start, here are four sites that experts say could be poised to lead the digital music revolution.

iTunes

Distinctive feature: No monthly subscription fees. 99 cents a song for burning and permanent PC downloading.

Apple Computer's (AAPL: Research, Estimates) much-hyped newcomer made its debut Monday. Unlike other digital services, iTunes offers selections a la carte sans a monthly subscription fee. Customers can download songs onto their hard drives or portable devices for 99 cents apiece. You can also preview 30-second sound clips for free before buying.

The service, available only via Apple's Macintosh computers so far, has a captive audience, according to Lee Black, a senior analyst at Jupiter Research, who says most other legitimate music sites are available on Windows-based platforms. But that's not necessarily an impediment to success. "You can have a small market and still make big bucks," he said.

Analysts will be watching iTunes carefully because of its potential to dominate the market. Plans for a Windows-based site, due by the year's end, could significantly increase the site's market share because it will be the only major site available on both platforms. Also, industry experts say, CEO Steve Jobs' ability to negotiate with record labels and artists could easily translate into more exclusive music for the site.

According to Apple, iTunes currently features exclusive songs from over 20 artists including Sting, Bob Dylan and Eminem, as well as tracks from artists such as the Eagles who haven't released music online before.

Rhapsody

Distinctive feature: Customized programming. Free seven-day trial; Radio Plus $4.95 a month; All Access $9.95 a month.

The site, owned by closely held Listen.com, is adding about 5,000 tracks a week to its 320,000-song library, said spokesman Matt Graves. Rhapsody also allows users to create customized play lists and access songs on multiple computers using a personalized directory.

The site, however, relies heavily on streaming, which may alienate dial-up users. Streamed songs play as they are downloading and dial-up services may not be fast enough to play the music smoothly. Also, Rhapsody does not allow users to save songs on their hard drives, though they can burn from streams directly onto compact discs.

RealNetworks (RNWK: Research, Estimates) has agreed to buy Rhapsody parent Listen.com for $36 million in cash and stock. IDC analyst Susan Kevorkian said Rhapsody's music catalog and service made it attractive to the Internet media company.

"RealNetworks' direction over the past couple of years has been more offering content via the Internet and less on technical innovation," Kevorkian said. "RealNetworks should be able to provide financial stability [and take] the service to the next level."

MusicNet on AOL

Distinctive feature: AOL Parental Controls. Basic service $3.95 a month; unlimited $8.95 a month; premium $17.95 a month.

MusicNet markets digital music services to distributors such as America Online, not to consumers directly.

Kevorkian said this strategy allows the service to focus on developing technology and relationships with record labels while the distributors handle marketing. The service has just two distributors thus far, AOL and Internet portal RealOne Media, but Kevorkian doesn't see this as a handicap.

"AOL has over 30 million subscribers at this point and they are developing their broadband services. That's a lot of eyeballs," she said.

Still, MusicNet's limited search capacities may drive customers elsewhere, some analysts said. "MusicNet isn't the best for discovering new music," Jupiter's Black said.

Unlike MusicNet, both pressplay and Rhapsody offer recommendations for artists similar to those you've already chosen, in addition to standard song searches. MusicNet's shareholders include Bertelsmann, EMI, Warner Music, RealNetworks and Zomba Recording Corp.

AOL and Warner Music are owned by CNN/Money's parent company, AOL Time Warner Inc. (AOL: Research, Estimates)

pressplay

Distinctive feature: Billboard music charts back to 1955. Unlimited $9.95 a month; Unlimited Plus $17.95 a month; Annual Plus $179.40 a year.

A joint venture of Vivendi's (V: Research, Estimates) Universal Music Group and Sony's (SNE: Research, Estimates) entertainment division, pressplay offers a wide range of features including streaming, downloading and burning, and boasts a 300,000-plus song library.

"Our site has been dedicated to innovation from the outset," said CEO Michael Bevel. "We were the first to recognize the need for portability and the first to launch CD burning," he said.

The service also allows users to discover similar songs and artists based on their preferences, and has premiered, or pre-released, songs for several popular artists, including Jennifer Lopez.

Bevel declined to comment on any new projects in the works, but experts say customers will need to see some new features soon if it hopes to keep up with Rhapsody and iTunes.

We'll let you decide that. Many of the services listed offer free trials, so check out the ones you're most curious about.

Online music users should also note that most digital music services -- including MusicNet on AOL and pressplay -- offer unlimited so-called "tethered downloads." Tethered downloads are "rented" files that not permanently stored on your hard drive. If you stop paying your bills, these files will disappear. Permanent downloads are usually additional.