I came across the wonderful Audioscrobbler a couple of weeks ago. Add one of their plugins to your audio player of choice and it’ll start compiling a list of what you listen to (mine is right here).
But the fun continues! They take that list and then use it to fuel Last.fm, a personalized net radio station. That’s just icing on the cake if you ask me.
(Hat tip to Designweenie for pointing me in the right direction)