When traveling, I am always amused to see other iTunes libraries appear in my iTunes as we all glom on to the same wireless network. "Oh, hello there, Sandy's MacBook, I didn't realize you were such a Hootie and the Blowfish fan." If you're constantly asking your friends about their iTunes libraries (excessively, perhaps?) you might want to check out Mashduo -- an OS X application that allows you to drag and drop library files for comparison. Finally you'll know who has the larger ABBA collection, or who never quite understood The Dead Milkmen (hint: they only song they own is "Bitchin' Camaro" or "Punk Rock GIrl").
As you can see in the video, Mashduo is pretty fast for what it does: collating and analyzing all that iTunes data. I cut about 22 seconds out as Mashduo did its thing to over 14,000 songs in those libraries. So yeah, it's fast. It is also a limited use piece of software, yes, but it is open source, so you can monkey around with it to your heart's content. First feature request: it would be lovely to have it work with those mystery iTunes shares over the network.