snychronicity

Unison File Synchronizer is a tool I make heavy use of at home. I'll let the author summarize:

"Unison is a file-synchronization tool for Unix [including OS X] and Windows. 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."
It runs from either the command line or from a GUI client and in my year+ of using it has lived up to the promises on the web site.

Other routes to accomplish the same thing are Bill Stearns rsync-mirror or rsync-backup. (Bill has lots of other neat stuff on his site; check it out.) There are also a number of various FOSS projects for mirroring and synchronizing out there, but their discovery and evaluation is left as an exercise for the reader.

(update) Harmony is another interesting project from Prof. Pierce. Having one set of bookmarks automatically sync'd between my various computers is something that I have wanted for a long time.