Duplicity is a backup program that only backs up the files (and parts of files) that have been modified since the last backup. Built on FLOSS (rsync, GnuPG, tar, and rdiff), it allows efficient, locally encrypted, remote backups.
The rsync utility is a very well-known piece of GPL'd software, written originally by Andrew Tridgell and Paul Mackerras. If you have a common Linux or UNIX variant, then you probably already have it installed; if not, you can download the source code fro
rdiff-backup backs up one directory to another, possibly over a network. The target directory ends up a copy of the source directory, but extra reverse diffs are stored in a special subdirectory of that target directory, so you can still recover files los