Last.fm

I've been checking out Last.fm over the last couple of days, and so should you. Basically, it lets you build a radio station that is totally customized exactly to your tastes. You end up with a radio station that only plays the music you like. In addition, it recommends new stuff to you based on your previous musical preferences and constantly updates based on new stuff you are adding to your library. It is totally cool, totally free, and totally tubular, dude! You should get online and spend a few minutes playing around on it.