A bootable backup!
Time Machine is a great backup solution, but it is unfortunately not bootable. It is really why alternatives like SuperDuper or Carbon Copy Cloner are still useful. The advantage of a bootable backup is that in case of a failure, you don't have to wait and resetup the whole thing, you can simply boot from the external backup (with firewire) and continue working. Carbon Copy Cloner can also be very useful if you are upgrading to a larger hard drive and you want to clone your current hard drive. Simply copying the file to the new hard drive won't work. You can download Carbon Copy Cloner here from MacUpdate.