Audioscrobbler monitors which songs you play on your computer, and uses this data to help you discover other users that share your taste in music. This data can also be used to calculate a level of similarity between artists and to recommend new music based on your listening history.
I've run a collaborative filtering algorithm over a snapshot of the database and used the results to produce an interactive TouchGraph visualisation of the links between related artists, which you can explore by double-clicking on the nodes of the graph.
Links from the info boxes attached to each node lead you to Audioscrobbler.com, where you can find out which users listen to a particular artist (and which other artists they listen to), and to Google for finding out further information.
Amazon tells you "people who bought this item also bought..."
Audioscrobbler tells you "people who listened to this artist also listened to...".