If You're Not on Spotify, I Don't Know What You're Thinkin'
Spotify is like iTunes + last.fm + Pandora... basically a revolution in social music. Okay, or maybe it's just a cool way to share music with your friends.
Either way, it's pretty addicting, considering I can now look up pretty much any song I want to hear and listen to it in full, not to mention I get to spy on my friends' fave tracks.
Download it for free here.