An easy-to-use and efficient way to compare iTunes libraries. It only shows the differences between two libraries, but if that's way you want, Mashduo does the job.
I try to keep iTunes libraries in sync between all my Macs, but somehow was missing a few items from a couple of libraries. I dragged the iTunes Music Library.xml files from two Macs into Mashduo, and voilá! It produces a list of media that are on each Mac but not on the other.
Note that I said "media." Mashduo doesn't distinguish between music, audiobooks, movies and TV shows. I would love to be able to select or deselect which media I want to scan for in the "advanced options" section. Other than that, Mashduo is a fine piece of work. Many thanks to the dev!