Wow - I remember hearing about this, maybe even posting about it awhile back, but I never got around to installing Audioscrobbler. But it kicks ass.
So basically it's a plugin for whatever music player you have (Winamp, iTunes, Rhythmbox, etc.), and it sends the track information back to the Audioscrobbler servers. It then compiles a list of everything you play and matches your musical taste with other users, AND gives you suggestions for other bands you might like to hear. I've only had it installed for a couple of days, but after a month or so I'll have a really good idea of exactly how often it is that I listen to certain things.
EDIT: I almost forgot (okay, I did forget) to post my own musical trends.