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.
SlingShot 2.0 adds support for synchronizing Playlists of music from iTunes. It works just like you might think: you Publish an iTunes Playlist from one computer, and then Subscribe on another. SlingShot will copy over any songs from the playlist that arent in the Subcribers...