While there are several products, one called SuperDuper has proved very useful. It does a very nice job of mirroring your system disk (or any other mounted volume) onto a FireWire or USB disk. It makes the copy bootable so if your system hard-drive is completely destroyed you can continue to work by booting off of the copy. Plus, it has a "smart update" feature which, like rsync, only copies what has changed since the last backup.

-- DarrellSchiebel - 30 Nov 2006