I use SuperDuper to backup my complete Boot Drive every friday, to two different locations. Both bootable. So at any given moment, I have a bootable copy of my OS on 3 drives. One is an internal drive, the other is an external.
I also then use ChronoSync to backup the data on my other drives. I use a Mac Pro, so I have 4 internal drives and 2 external drives.
the combo of the 2 basically gives me pretty crazy redundancy. I have it like this:
Boot backs up to seperate internal drive and external2 drive every friday
Any files that are important back up to external1 every night. The same files also back up to external2 every friday.
Basically, I have a nightly external, and a weekly external. ChronoSync has been awesome.
Oh, also, I have a syncronizer profile I use to sync my Mac Pro files to an external USB drive, which then I sync with the same files on my iMac at home. By doing this, my iMac and Mac Pro have the same work files on them anytime I plug in the USB drive. ChronoSync is set to sync the USB drive any time the drive is plugged in.