We're a music player that lives in your browser for your personal music library. We cross reference data so that if one user comments on "Metallica Enter Sandman", then all other users playing that song will see your comment.
It also shows who's playing what, some basic social filter, rating of songs etc.
There's a lot of work to be done, and it is all Flash based (sorry), but it's very highly functional.
It does require installing a small program on your PC to stream your music, but you can at least see the site and some of the basic functionality without doing that.
We're open to suggestions.