I don't know if anyone's already seen this site or not, but it's really very cool indeed.
Basically, last.fm allows you to create a personal music profile (totally free) based on the stuff you listen to on your PC. It works by running a plugin for your music player of choice (Winamp, Windows Media Player, iTunes, etc.) and reporting the tracks that you listen to into a central database.
The upshot of this is that they will generate weekly and all-time charts of your listening preferences for you, and these get combined to create weekly charts covering all last.fm users. You can also create/join groups and have group charts too.
On top of this it will also point out people who have similar listening tastes to you so you can check out what other stuff they like, and recommend similar artists. There's also a streaming radio included, which will play only stuff you might like, etc. Very neat.
Anyway, if you want to see what a user profile looks like, here's mine (been a member coming up to 2 years now):
http://www.last.fm/user/Mu77ley/Muttley