Last.fm
I’m very intrigued by Last.fm, formerly Audioscrobbler.
Download a plugin for your favorite music player (Linux players included!) and start listening to music. From there Last.fm will start recommending music to you based on what you and others listened to that’s similar.
I can’t do justice to explaining it, so go read the FAQ. I’m very intrigued in Last.fm as a way to get introduced to more music, but not sold on using XMMS as my music player as I love Muine and am starting to test Banshee.
But I love the concept of Last.fm. Social networking tools own me.