The Unison is a nice utility to keep the directories on the same computer or different computers.
Thanks to the use of a rsync-like protocol, the synchronization time is proportional to the size of changes.
It does not require starting any additional services.
If the remote machine is accessible via ssh is has the unison installed then the unison will be able to synchronize these two machines.
It has simple settings and a clear interface (both common-line and GUI).
Unison is cross-platform and works across Linux, Mac OS X, and Windows.
It has a plethora of uses. I use it to: