Spotify For Windows Phone 7
"The app itself looks incredibly slick, and is ultra snappy to use. Spotify catered the music streaming experience elegantly to Windows Phone's Metro User Interface, and includes all the features you'd expect, plus more."
The Spotify music streaming service is not available to everyone, but if you are able to connect to it then this app for Windows Phone 7 should be on your list. This is a very nicely-designed Windows Phone 7 app. Features include playlists, a news feed, "now playing" indicator, and a "People" screen that helps you pick out a friend and see what he or she is listening to. Well worth checking out.