This morning I stumbled upon a neat looking program called MusicIP. It's a tool for analyzing your music library. Once analyzed you can select a song and make a playlist based on that song. You can also do things make a new playlist based on a current playlist (though that seems to require some form of registration that I haven't figured out yet).

The only thing that is a pain about it is that I have a fairly large library in iTunes (around 6,500 songs) and so the analysis phase will be going on for ages (fingerprinting the tracks will take about 20 hours and analyzing the tracks 42 hours!). But so far with just a fraction of my music processed it gave some good results. When I've gotten more of my library processed I'll post an example one