The relationship between music and the Internet is a strained one. What with piracy and illegal file sharing, people always seem to cringe when the two come together in a way that doesn't involve a credit card as a mediator.
Last FM is a breath of fresh air in this foggy haze of legal paranoia. It monitors what you listen to and uploads your preferences to your profile, visible to all. You can then connect to others with similar choices or even find music similar to what you've been listening to.
Then you can use the downloadable player to listen to radio stations featuring the music you like. It's all custom tailored to you based on the music you're already listening to on your PC.
While I'm not a big music person, I can see huge potential in this type of music/social networking. The site even includes a Wiki with user editable information about artists, tracks, and albums.
Here's hoping it gains popularity!