Pandora Radio (Pandora.com)
You don't have a lot of control over what you listen to, but it gives you a ton of new bands based on the band/bands you already like. It helped me discover a  ton of awesome new bands, and it's free - which is just the buttercream frosting on the delicious chocolate cake of what it is. :3
Then again, I don't know a lot ( Read more... )