Streaming music services like Pandora and Slacker are a hit, but they've left consumers wanting more. Cloud-based subscription music services that allow you to play any song you like whenever you like, over and over (even that awful Barry Manilow tune you've been humming in your head) are gaining traction.

While Apple still hasn't let on what it plans to do with its recently acquired Lala music service, iTunes users are hungry for a streaming option. Recent research from NPD Group found that out of 50 million iTunes user, 13 million to 15 million would be willing to pay $10 per month for an unlimited streaming option. That translates to somewhere around $1 billion in revenue in the first year.

If and when Apple gets around to evolving iTunes with a cloud-based streaming component, there's a growing field of capable competitors already out there. Here's a look at what $10 a month will get music listeners right here and now.

Rdio (Total Advertised Catalog: 5 Million Tracks)
While not available in the United States yet, Rdio is coming soon and it offers an impressive service. We got a test drive of the service and found that the service has potential to be a full-on contender in the subscription-based music game. The company claims access to "5 million songs" for a monthly fee of anywhere from $4.99 to $9.99.

The $4.99 price tag allows users access to the online service, whereas the $9.99 service includes mobile streaming, as well as the ability to cache a certain amount of favorite music to the device for offline listening. 

Through a unique UI, Rdio offers deep integration with social networks Facebook and Twitter, as well as social music service Last.FM, so you can share playlists, songs and recommendations with others. There's even a 'follow' option, similar to the way Twitter works, where you can view what people in your community are listening to.

While some may confess to burnout on the social networking scene, it's an interesting complement to streaming music services. When you're presented with a catalog of 5 million songs, recommendations from other listeners can be a great way to discover those diamonds in the rough you'd have otherwise missed.

Rdio currently offers apps for iPhone and BlackBerry, with an Android version on the way.

Rhapsody (Total Advertised Catalog: 9 Million Tracks)
If Rdio is the little record store around the corner, Rhapsody is the Wal-Mart of streaming music services. While Rdio can be used with just an app and an online service, Rhapsody includes a bulky, slow-moving desktop software that has tie-ins to a host of Microsoft's media software and services, as well DRM and the like. Rhapsody wants be your music hub, just like iTunes, and as a result it feels big and cumbersome and at times, limiting.

As far as discovery goes and offering at least a modicum of interaction with other listeners, Rhapsody allows users to create and submit playlists, in much the way allows users to create and submit reading lists. It's a far cry from the kind of unique integration seen over at Rdio but it's at least a small carrot as this behemoth goes on trying to keep stickiness to a maximum.

Note that Rhapsody does allow users to share songs by pressing a Facebook, Twitter or email icon but it is simply not the sophisticated community-building system that Rdio has in place.

While Rhapsody can at times feel like too much, it still boasts a larger catalog than some of the smaller services like Rdio, as well as a suite of other services like its own radio (similar to Pandora and Slacker), where the user can create stations by specifying an artist. Although Rdio may be a bit more of a manageable service, it's hard to argue with Rhapsody's 9 million songs. Given that Rhapsody has been around the longest, at least in the United States, they've had time to negotiate strong licensing agreements with a lot of major labels, and that means they're getting more of the new releases sooner than a new comer. 

Rhapsody currently offers mobile apps for BlackBerry and iPhone and is available as a paid service through Verizon Wireless' V Cast service.

Thumbplay (Total Advertised Catalog: 8 Million Tracks)
Thumbplay has promise but lacks focus and refinement in its current beta version. While it's $9.99 per month price tag includes 8 million songs, the service as a whole is still a bit shaky, as is to be expected with any beta product. Thumbplay's only real differentiation between other services is it's attempting to closely resembled the cloud-based streaming music service that Apple has yet to conceive.

Separate desktop software, as well as a mobile app (Android, iPhone and BlackBerry), need to be downloaded with Thumbplay. At launch of the desktop service, users are immediately asked if they want to import their playlists and favorite artists from iTunes. That idea is nice but it's not enough to carry the service.

Thumbplay's UI is clean and neat, but the logistics of managing music were by far the most confusing of these services. The use of playlists as a way of saving an album to your library works alright but as Rhapsody and Rdio demonstrate, it could be made easier by just clicking a button next to the album.

While there is a way to authorize your Facebook account, that option is not front and center and the social component is not emphasized at all. Another thing that bothers me here is the home screen interface in Thumbplay's desktop module is rather sparse, which means there's very little in the way of recommendations (picture all the flashy recommendations in iTunes music store and you'll get an idea of what Thumbplay is lacking in this department).

Thumbplay's selection is competitive with the other two services even when venturing into more obscure territory, but the overall experience is still a bit buggy (the desktop app crashed quite frequently). But perhaps this service's biggest downfall is it lacks offline listening. Thumbplay says the feature is on its way, but until it's added, Thumbplay simply isn't playing in the same ballpark as Rhapsody and Rdio. 

That said, Thumbplay's impressive catalog and existing services feel like it has possibility. If the company can offer offline listening and smooth out some of the rough edges, it could eventually contend.

MOG (Total Advertised Catalog: 8 Million Tracks)
MOG, which stands for Music On the Go, is yet another subscription-based music service which recently launched mobile apps for iPhone and Android. For $9.99, MOG users can download or stream unlimited tunes from the company's catalog to their mobiles.

The UI on both the app and the online version of the service are easy to use, but MOG offers a slightly different approach than Rhapsody. Where Rhapsody makes streaming music easier than downloading music to the device, MOG seems to put the emphasis on downloading to the device. It's a matter of preference, but with MOG it seems like users will more often find themselves storing music on their device. That's good or bad depending on how much available space you have on your device.

MOG's real strength is in its extensive "News and Reviews" section, which is broken down into categories (i.e. Pop, Rock, Alternative). These pages offer constantly changing reviews of new artists, albums and trends, as well as rundown of various news items from around the music industry.

Additionally, MOG offers news, photos, biographical info and a place to post comments about any particular artist. There are also buttons to share on Twitter and Facebook. The overall system is a good way of developing that sense of community that Rdio has succeeded in creating, and it goes a long way toward giving users a rich discovery experience.

MOG's radio offering is also well done. Once a user has specified an artist, he or she has full control over the station. You can skip ahead or go to previous tracks in any given radio playlist, which is yet another great way to discover new music.

MOG's catalog is impressive enough but for some reason Rhapsody still feels like the most polished of the services we tried. MOG scores points for well-executed social and radio offerings.

Is $10 Per Month Worth It?
Short answer: Yes. When I think about how I listen to my music, it is almost always on my phone or iPod touch. If I'm not plugging my smartphone into the home stereo to listen to my latest music purchases, then I'm plugging it into the car stereo. Aside from being able to occasionally "share" a CD with a friend, there's very little that music listeners will be missing by having their collection stored on the cloud. As long as offline listening is allowed, there's value here. It's worth mentioning that the sound quality in all three of these services is excellent.

The marketing ploy used in the subscription-based business model is that $10 a month equals what a person would spend on one CD per month, which is true, but in the case of subscription music, you get millions of songs for your listening pleasure, as opposed to one album.