I've started a web-application that uses similarities between users' songs (the ones they like) to give a recommendation of songs they would probably also like.

You can find it here: http://musicmatch.prijsindex.net/

Give it a try, you might like it.

One thing: only 49 users as of yet, and the system gets better as more people join, so it can use all the help it can get, but eventually your input will pay itself back greatly (if you tell enough friends about it ).