Check out
http://www.pandora.com/ for a simple idea done in a really cool way.
You enter a bunch of your favourite songs or artists into the music player to create custom streamed "radio stations" that include songs based on your musical preferences. As each song is played you can give it a thumbs up or down to refine the suggestions - as the site says, it's like someone saying "Oh, if you like this song then you should listen to this". It's very cool, and also very effective, as it's playing some damn fine songs for me.
Sometimes, inbetween all the crap out there, you find a really cool, useful feature on the Internet, adn I think this is one of those times.
