www.pandora.com
You put in the name of a band or song you like, and it plays other stuff similar to it. You give feedback on the music it plays, and it edits the playlist from there. Very helpful for discovering new music.
Of course, it does have a few flaws. Sometimes, it plays stuff that you wouldn't think would go together (I put in Dark Tranquillity and get Godsmack). The guys that run it say it's not a flaw (check the FAQ for their reasons why, but I still disagree).
You have to register to use it after the first time, but it's free, so don't worry.
Anyone else use Pandora? 
