I play my tunes through Winamp and have the Last.fm plugin installed so it uploads my track data for each song I listen to to the site...then the last.fm site recommends similar music based on what other stuff other people listen to who also listen to what I listen to...

Works pretty well, but one day I would really like to do a mashup that better allows someone to start at a certain band and branch away to newer artists as they explore similar bands. Just not enough time in the day....