Turntable.fm makes waves, but what is it?
The service is this: you sit in a graphical chat room, upload songs to a queue (which feels naughty and, for some relatively complicated reasons, is, outside of the U.S.), and those songs play, in a round robin between you and four other "DJs." That's about it. From there, you collect fans like Twitter, and points, based on how subjectively rad your taste/talent for feeling out the vibe is. If not, your song will be voted down and skipped (harsh). The points are a subtle, addictive thing, allowing you to purchase avatars that visually represent how cool you are -- it's a video game mechanic as old as the stars, but predicated on people's validation of things you already like. It's not hard to get sucked in... the damn thing is fun.
The site is open to anyone with a Facebook friend already connected to the service, a semi-closed beta-type situation that has already attracted 140,000 people. It's begging to be used for promotional purposes, as Talib Kweli recently did, and as Diplo and Hipster Runoff attempted to do (they were thwarted by the general disposition of the internet, naturally).
While it may not be a Pandora-killer, it's definitely not as bad as some half-baked internet startups of late, and will likely be getting some major funding, and some significant branding and advertising, very soon.