Last.fm

I registered at last.fm last week.
Last.fm is a website that keeps track of the music you listen to, using plugins for the most used music players around. When you listen to your music, with your favorite music player, information about the songs you have heard, is sent to the last.fm website.
This information is used to make a profile of your listening habbits: what is your favorite artist, what kind of music do you like most, ...
This profile is then used to match you with other people who have a similar taste. You are provided with other music those people like and which you migth like too, as you have a mutual musical taste.

At least that's what the website is telling me. I have used the plugin for a few days now, but it will take a few weeks - according to their website - to build a profile.

BTW: I got the plugin working on iTunes (it keeps track of both the music I listen to on iTunes and my iPod) and on XMMS (Linux).