Lots of people know--and have grown to love--Last.fm, which has over 5 million music tracks available to listen to for free online. The Last.fm mobile client
lets you take all of those songs with you (wherever you have 3G coverage) and enjoy them, commercial-free, for hours.
The more you listen, the more the app can learn about your tastes in music and deliver appropriately personalized recommendations. While listening, you can check artist bios and concert information, and you can even let your buddies know if you’re attending a show. Last.fm for iPhone also syncs your playlist with its Web site so that you can keep track of your listening habits.
If you like Last.fm, you might also want to take Pandora Radio