Pandora

I just recently signed up for Pandora. It's free and it's pretty damn cool! You type in songs or artists that you like and Pandora selects music based on that first entry. As each song comes up you can vote if you like or dislike each one. If you like, Pandora remembers that if you don't like Pandora stops playing that song and starts a new one. It's actually really neat. Best part is there are no commercials! If you come across a song that you like enough to want to buy Pandora has an option to "buy this song in iTunes", how cool is that? I'm getting ready to get some house cleaning done and I have Pandora playing right now. Right now it's Inxs, Need You Tonight and I first chose Depeche Mode as the artist and from there Pandora plays songs based on what I like and what I don't. 

Highly recommend!

http://www.pandora.com