last.fm
A mainly user-defined website. Like MySpace but way better. Last.fm uses a plugin to create music charts based on what one listens to on their (computerized) media player. Based on these, it recommends music and creates radio station. Users who sign up for a free account receive these benefits, and can also post blog entries. A good way to discover new music and meet people with good musical taste. 

Also known as the "Social Music Revolution"
Person 1: Hey, why do your last.fm charts list Lindsay Lohan as your No. 1 artist?
Person 2: It was my kid sister, I swear.