Spotify-WP7Music streaming startup Spotify has launched an application for Windows Phone 7, a move that will help it maintain momentum across the web and mobile platforms.

Spotify joins fellow on-demand streaming services Rdio and Rhapsody, which also offer Windows Phone 7 apps. While Spotify has captured the most headlines, it has lagged in the mobile space by not fully fleshing out its iOS and Android apps. It also recently launched an app for a limited number of BlackBerry models. But the Windows Phone 7 app looks like it may be the best mobile app for Spotify yet.

Using Windows Phone 7’s stylish design process with large headers, clean lettering, lots of white space and nice looking photos, the Spotify app lets paid subscribers listen to as much music as they want from Spotify’s more than 13 million songs. Users can stream songs over 3G and Wi-Fi, can access songs offline with caching, and tracks are synced whether you’re using an account from your phone or computer.

To legitimately use the app, you must have a premium subscription to Spotify, which runs $10 a month for mobile access on top of PC and Mac access.

Spotify has seen substantial growth in its revenue and number of paid subscribers since its launch in the U.S. in July. As of October, the service has more than five million active users, with two million paying subscribers. Spotify has raised about $120 million in funding so far and is valued around $1 billion. It raised a round of financing earlier this year from Russia’s DST, the investment company that has previously backed Facebook, Groupon and Zynga.

You can see more photos of the slick-looking Spotify WP7 app below: