While MediaRover is billed as sync for the entire family, it works all the same if you’ve just got multiple machines yourself. If you add a song to one computer, other networked computers get it as well.
Beyond backup, this is great for streaming to your Sonos, XBox 360, PS3. In addition to syncing music, it pushes metadata updates as well. Any changes you make in one library will sync right up with all your other libraries.