rsync is a clever algorithm for synchronizing files in a way that is bandwidth efficient. It was invented by Andrew Tridgell and Paul Mackeras. It is described here:
http://rsync.samba.org/tech_report/
rsync is also a program that implements the algorithm.
NasBackup [L1 ] is an Open Source Tcl/Tk application that provides a GUI layer on top of rsync.
CMcC notes that rsync can be used to backup a frequently changing database, for example by:
rsync rsync://wiki.tcl.tk/wiki/wikit.tkd .
One would have to repeat the rsync until no changes were sent, though. Is there some return value from rsync that tells you how much work it had to do?