Turntable.fm is the latest social music engine and so far it's pretty cool. Let me tell you the basics.
It's similar to blip.fm in that when you join, you become a "DJ" and you get to play songs that you can find on the search. You can also upload your own mp3 files and so you have the option of sharing your own music.
You can create a room, or join a room and this is one key difference from blip.fm (if you're familiar with the service). The room includes a space where your avatar hangs out and can listen to music, or you can become a DJ and your avatar gets put on a turntable and you can start sharing your music.
It's a very engaging platform for several reasons:
You add songs to your Queue and everyone that is DJ'ing gets a turn.
Songs are voted "Lame" or "Awesome", too many lame votes skips the song
Five DJ's have the spotlight for a room, everyone else can listen, if you choose to DJ, you have to keep picking good songs.
There is a chat feature that actually works.
There's gamification because you can get points and levels and you can buy upgrades with your achievements. Check it out, my