Mashduo lets you compare your iTunes music with all of your friends. You just need to drag and drop in you and your friend's iTunes library files, and Mashduo will compare them. You'll get a report of all the songs you two don't share. You can then export to a file or email the list to your friend. You can even zip up all the songs your friend doesn't have, so you can let your friend sample them.
You can download Mashduo right here. Mashduo is also open-source. You can download the Mashduo source here. If you don't have anyone to compare libraries with, why don't you compare libraries with us. Here's our sample library.