zbackup is a globally-deduplicating backup tool, based on the ideas
found in rsync. Feed a large .tar into it, and it will store duplicate
regions of it only once, then compress and optionally encrypt the
result. Feed another .tar file, and it will also re-use any data found
in any previous backups. This way only new changes are stored, and as
long as the files are not very different, the amount of storage
required is very low.
