Pandora
I was recently introduced to Pandora by darxus; you can read his post about it, but basically, you tell it music you like, and it plays similar music. For free, over the net. And you can give it as many different seeds as you like, and keep them separate, so you can listen to fast bluesy jazz instrumental stuff today (what I’ve been listening to today) and Europop tomorrow. I think this site is going to seriously affect how (and how much) I listen to music.