A few of my friends have recently introduced me to a website called Last.FM. The idea is quite simple, though I suspect that the mechanics behind it are quite complex. The basic idea is that as you listen to music, your computer uploads the title of the tracks you're listening to to the website and slowly it gets to build up a profile of the kind of music you like. The site then compares you to other users and starts recommending music to you. I have yet to build up enough information to get anything other than Queen, U2 and the Rolling Stones recommended, though I suspect most new users will get them.
It also allows me to make the latest addition to this website, the little "Recently listening to" box on the left hand side. It doesn't always update too quickly, but gives anyone who's interested a bit of a sneak peak into what my music tastes are. Actually, it tells you what I listen to while I'm working. I tend to have quite different tastes in the car, for example.
I can be found on Last.FM as prlewis.