pandora.com
This thing is tight. You type in the name of the song or artist and it'll make a personalized station for you based on that sound.
You can type in "Prodigy" and get a library of old school electronica/rave or say "Smack My Bitch Up" and get songs that have a specific sound to it.
It's free but will be supported by ads in the future.
This is all I ever listen to now.