There's been a bit of a buzz about Turntable.fm over the last week or so. Currently it's in private alpha - you can only get in if you're Facebook friends with somebody who is already in - but if you can get access you should definitely check it out.
I've been waiting for a site like this for years. It's very, very simple, and that's the beauty of it. You form a chat channel (or join one), and you're entered into a room where all your avatars gather around on a dancefloor. There are five DJ spaces, and people can take turns DJing on the platform. You have a playlist on the right hand side of the screen where you can arrange the tracks you want to play when you DJ, and when you're just listening you vote on whether or not you like the track that's being played. When people like it their avatars' heads start bopping along to the music, and if too many people think your song sucks then it gets skipped.
If there's a good track on people will start talking about it, and it's a really cool way to find new music. The service already has a lot of music uploaded, but if you want to play something and they don't have it you can upload it. Only you will be able to access it if you do that, so you don't need to worry about people stealing things if you're playing your own creations.
I could rave about this service all day, but I won't. Instead I highly encourage you to go check it out; hopefully you'll be able to get in, and if not I'm sure at the rate this is growing that it will only be a matter of time before you know somebody on the inside. Enjoy.