pandora.com

I'm sure that for some of you this is old news, but for others it may revolutionize your music life.

Pick a song, artist, or genre and get a radio station composed of others songs pandora thinks you might like. You can 'like' or 'not like' any song you hear, and the station will further attempt to customize itself for you. I think it does a pretty good job and use it all da time.