Mac OS X only: Shareware application Syncopation keeps your music, podcasts, and video in sync between up to five iTunes libraries. The app finds music that's in one library but not others, then transfers files between computers until everything matches. Syncopation is smart, too: it automatically replicates changes to a track's metadata in one library across your others. You can even set up block lists that keep, for example, your thrash metal out of your significant other's library. Syncopation is $25 for the full version, but you can download a demo version that works with two computers for 30 days—so if you just want to run one quick sync between two libraries, it's an excellent free solution. Either way, the Mac OS X-only Syncopation is a great tool, though it'd be great to see a completely freeware alternative.