LastHistory - interactive visualization your Last.fm history, obviously
Takes a while to initalise the first time, but once the data is in there it's loads of fun to play with. It's also got a really nice 'personal mode' that hooks up to iPhoto to display pics taken at the time you listened to the music.
It's free to download and try for yourself: http://www.frederikseiffert.de/lasthistory/
