Playdar is a content resolving service for your music.
It's essentially a semantic web agent - but with no RDF around to add in that 'semantic' part.
A website asks your browser for permission to play tracks, much like Songbird offers, and it allows you to index / search your music.
More importantly, it's possible to search your friend's music, auto-magically, and stream from there. Or at least wired claims so.
I'll wait and see if it ends up more polished.