I have recently been forced to learn how to admin an OpenSolaris machine with ZFS filesystems. After doing some reading on ZFS, I realised how using snapshots with an rsync server would make an excellent backup solution.
Using several scripts to control rsync and the snapshots has allowed me to setup a system that has data retention periods applied and only uses the space on disk required to keep the changed files. Cloning a ZFS snapshot from a week ago (or even six months ago) allows you to create a synthetic full backup from the point in time the backup was taken.