rdup is a utility inspired by rsync and the Plan9 way of doing backups.

rdup itself does not backup anything; it only prints a list of the names of
files that have changed since the last backup. It also handles files that are
removed, allowing for correct incremental backups.

An example script named rdup-simple is included to show how to 
implement a simple snapshot-style backup strategy.

WWW: http://miek.nl/projects/rdup