Mac: Sharing music with friends and significant others is an age-old way to spend a few hours. Mashduo makes the process a little easier by analysing two iTunes libraries and dishing out a report of all the songs you don't share.
Mashduo only requires the two XML files from the iTunes libraries that are going to be compared (~/username/Music/iTunes/iTunes Library.xml). Drop both files into Mashduo, and it generates a report of all the songs you don't share. You can then load up the results in a text file, XML file, or zip up all the songs on your computer your friend doesn't have so that he or she can easily check them out. If you share a lot of music with your friends, it's a handy app to have around.