Pandora Radio All of what makes Pandora great, now on your phone. The streaming quality over 3G is excellent. I once drove from LA to San Diego with Pandora playing the whole way without a single skip or loss in quality.
Last.fm Similar to Pandora, but the sound quality isn't quite as good on 3G
Ghostly Discovery Nice UI idea: set your mood, whether you want "organic" or "digital" and the tempo you're interested in, and you'll get a playlist of Ghostly International artists. You may not know the music, but that's part of the fun.
Slacker Radio Similar to Pandora and Last.Fm, but a little more focused on genres.