The Audioscrobbler plugin for Winamp/XMMS aggregates listening habits by tracking the artists/songs you've listened to, and comparing that information with all of the other users in the database. If this catches on, a system like this would be a really effective way to discover new artists and find people with similar tastes. (My personal stats.)
A simple idea, but brilliant. (Thanks, Cameron.)
Update: Audioscrobbler's service provider took the site offline because of increased bandwidth usage. The site's creator is looking for a new host.