"Playdar is a music content resolver service - run it on every computer you use, and you'll be able to listen to all the songs you would otherwise be able to find manually by searching though all your computers, hard disks, online services, and friends' music collections." Feels a lot like Audioscrobbler did when that first launched; it'll be interesting to see what user-friendly services get wrapped around it.

Visualisation for looking at your listening history on last.fm. The outputs are beautiful.