Unison File Synchronizer
This is the best free file synchronizer out there. "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."

Here's the coolest part: "Unison runs on both Windows (95, 98, NT, and 2k) and Unix (Solaris, Linux, etc.) systems. Moreover, Unison works across platforms, allowing you to synchronize a Windows laptop with a Unix server, for example."

It's bandwidth friendly and the connections can be easily tunneled through ssh.