Turntable.fm is maybe the most fun I've had with music since I quit dropping acid. OK, that's an overstatement. But it's quite fun. Here's how it works.
When you enter the site you can go into one of several rooms. Some will be packed, others empty. You can also create your own room. Once in, you can either jump on the decks and start playing music or just hang out and listen. If a song's not already in the Turntable database, you can upload it yourself. There's a chat interface, and DJs can earn points from the audience, which earns them both points and gear for their avatars.
OMG! Nerdy! Yeah. It's actually even nerdier than it sounds. But it's also a pretty nice interface to kick back and listen to music from your friends and discover new songs. There's even a Gizmodo room. You should totally drop in.