After two years of waiting, the U.S. market finally gets a chance to try out Spotify, the music-streaming service that's now wildly popular Europe. But why is there so much hype around Spotify, you may wonder? Don't we already have plenty of music streaming services here, like Pandora, Last.fm, Slacker Radio, MOG and Rdio, for example?

Sure we do. But we've never had anything like Spotify before.

Spotify got our attention at the SXSW conference in 2010 with a planned launch for the third quarter of last year. But as we all know, that launch never happened. The delay was caused by the company's failure to finalize deals with U.S. record labels - labels which were hesitant to allow any service to stream their artists' music for free. But in January, Spotify at last inked a deal with Sony, and earlier this month, rumor had it that the last holdout, Warner Music, signed as well.

Why is Spotify Different?

What makes Spotify different from its competitors, both free and paid, is that it offers you an option to play all the music you want on-demand, for free. The free level of the service is supported by ads.

That's a big difference from something like the free services provided by Pandora or Last.fm, for example. With those and others like them, you tell the service an artist you like and they'll build a customized radio station around that selection featuring that artist's tracks and those from similar artists.

But to actually stream any song you want, at any time, you had to pay. The rates were typically $5/month for uninterrupted streaming on your PC and $10/month for streaming to any device, including mobile.

Spotify offers those same paid options, too. But unlike the competition (e.g. premium services like MOG, Rdio, Rhapsody, etc.), it goes the "freemium" route to build up its user base. What that means is that instead of offering a brief, free trial, Spotify actually offers the entire service for free, for as long as you want. With a freemium business model, the hope is that as you continue to use the service you'll eventually desire the paid-only features that only a subscription plan can offer.

On Spotify, these upgrades include ad-free listening, offline access, mobile applications, enhanced sound quality, exclusive content and more.

Spotify's BIG Music Catalog

There's another reason why Spotify is intriguing - its catalog. If iTunes is the gold standard for digital music, now with 18 million tracks, then Spotify comes a close second with 15 million songs.

In May, we looked at several of the other digital music catalogs, and found that Spotify, then at 13 million tracks, was already far ahead of its competition. At that time, Slacker had 8 million songs, Rdio 8.5 million, MOG 11 million, Napster 10 million, Rhapsody 10 million and Grooveshark 6 million.

If anyone can compete with iTunes, it's Spotify.

On PC & Mobile

Spotify also has another trick up its sleeve - cross-platform access. While iTunes requires you buy from iTunes, getting that music copied onto non-Apple devices takes special software, like the (third-party) DoubleTwist player for Android users, or Microsoft's Windows Phone 7 Connector for those using Windows Phone.

With Spotify, the company offers native clients for nearly all platforms, including both Mac and Windows, as well as iPhone, Android, Windows Phone, Palm and Symbian. (Sorry BlackBerry.) And even better, there's no copying involved. Spotify's music is stored online, "in the cloud," not as physical files that have to be saved to the hard drive of your device. (The only exception being with offline listening - then, files would need to be saved locally).

Even with Apple's forthcoming iCloud service in iOS 5, it's still about storing physical files to your Mac, iPhone, iPad or iPod. iCloud is just the go-between that makes the syncing happen. With Spotify, you're streaming, not syncing. And the cloud, not the iCloud, is the future of a hyper-connected world, Spotify would say.

Get an Invite!

Spotify says, via this morning's blog post, that U.S. users can now sign up for an invite to the service. Those who can't wait can jump ahead in line by signing up for a premium offering instead.