http://www.pandora.com/ and the link police can eat my poop.
Ok, now probably everybody knows about Pandora, and I'll be welcome to some time ago with no mercy, but still...
Basically, it's a flash-based internet radio that tries to feed you with the music you like: you create a station by entering the name of an artist/song/album that you like, and it hopefully streams music similar to the one you've entered. You can mark the songs that don't fit your liking in order to fine-tune the station.
I'm getting addicted to the free service, let's hope they don't cancel or limit it anytime soon.