Music Library Sync for Mac

From Amphonic Designs:

Music Library Sync simplifies the process of maintaining separate iTunes Music Libraries on multiple computers. It is the easiest and fastest way to import music, movies, TV shows, and podcasts into iTunes from other iTunes Music Libraries. Music Library Sync delivers a list of media found in the remote iTunes Library that is missing from the local libray and allows you to select the tracks you would like to import. It is a great way to make sure your desktop and laptop computers both have your entire music collection without having to deal with duplicates and missing tracks.