Last.fm is a 'music networking' site, which works by looking at which music you play through your computer (e.g. on iTunes), then uses this information to build a profile of your listening habits.
This profile is then compared to that of other people, to see who has similar tastes as you - tracks or artists can then be recommended, clips or full-length preview tracks can be listened to, etc.
Think of it as a MySpace for musos...
Needless to say I've uploaded some BITL stuff, which you can listen to right now by scrolling right down to the bottom of this page & hitting 'play', you lucky people!