Unison File Synchronizer

http://www.cis.upenn.edu/~bcpierce/unison/

Unison is a file-synchronization tool for Unix and Windows. (It also works on OSX to some extent, but it does not yet deal with 'resource forks' correctly; more information on OSX usage can be found on the unison-users mailing list archives.) It allows two replicas of a collection of files and directories to be stored on different hosts (or different disks on the same host), modified separately, and then brought up to date by propagating the changes in each replica to the other

Discovered via closed group file-sharing summary on taint.org. This looks like a must-try to get files back and forth and round and round among the systems I use for work.