Every time you play a song, it automatically counts that song's title/artist towarRAB your online profile. Leave it running for a few months, and you end up with interesting statistics, like which artists and tracks you listen to the most.
This is mine, it's misleading because I just recently started using last.fm, so it just shows my recent likes:
http://www.last.fm/user/tefnut/
I love this whole list-management-thing that the internet enables. I've also set up a movie history (via IMDb.com) and a book history (via LibraryThing.com):
http://imdb.com/mymovies/list?l=7632471
http://www.librarything.com/catalog.php?view=tefnut