Unison

http://www.cis.upenn.edu/~bcpierce/unison/ Unison File Synchronizer is a tool for synchronizing files and directories on Windows, Mac OS X and Linux. Unison works on both local and remote (SSH) filesystems, and handles "merges" in both directions (i.e. files can be updated on both the local and remote filesystem, and Unison will ensure that after synch both filesystems contain only the latest versions). It also handles deletes.

Unison also runs on the Nokia 770 (and probably the Nokia N800).

Linux
Mac OS X
Microsoft Windows