Audioscobbler

I suppose that it was only a matter of time, in this era of ever more sophisticated targeted marketing like Amazon.com's 'people who bought this also . . .', that something like Audioscrobbler would pop up.  (I haven't tried it and don't know about the hassle involved, but Nora, who pointed me to it, swears that it's pretty cool.)

"Audioscrobbler is a computer system that builds up a detailed profile of your musical taste. After installing an Audioscrobbler Plugin, your computer sends the name of every song you play to the Audioscrobbler Server. With this information, the Audioscrobbler server builds you a 'Musical Profile'. Statistics from your Musical Profile are shown on your Audioscrobbler User Page, available for everyone to view.

There are lots of people using Audioscrobbler, but you probably won't be interested in most of them. The Audioscrobbler Server calculates which people are most similar to you, based on shared musical taste, so you can take a look at what your peers are listening to.

With this information, Audioscrobbler is able to automatically generate suggestions for new songs/artists you might like. These suggestions are based on the same principles as Amazon's "People who bought this also bought X,Y,Z", but because the Audioscrobbler data is what people are actually listening to, the suggestions tend to make more sense than Amazon."