Last.fm
I've been an avid Last.fm user for a while, ever since my friend Matthew introduced me to it.
Basically, it uses a plugin in your music player (iTunes) to keep track of what you listen to and how much. It then not only reccomends you new music based on what you apparently like, but pairs you up with "neighbors"; people with similar taste in music, giving you more to explore musically. It also lets you keep track of what your friends are listening to.
As you can see, I've been listening to a lot of Ted Leo (between the Pharmacists and Chisel - his band from the '90s), and I recenly got into the Hold Steady. Also, Mistah FAB has a wonderful place on my Last.fm playlist.
Anybody else on Last.fm can feel free to add me as a friend.
EDIT: Apparently that table updates itself every week with my most recently listened music. Sweet!