I maintain a couple of boxes that run Mac OS X Server. One of the great things about it is that I can use SuperDuper! to clone volumes. For example, I can take a snapshot of the entire boot volume before I apply software updates, and if something goes horribly wrong I can roll back to the snapshot. Or I can clone to an external drive and then swap in higher-powered hardware.
I also use SuperDuper to clone my MacBook drive to an external drive, and if my internal drive goes boom I can just boot from the external drive and keep working.
I imagine that a tool like this must exist for Linux or FreeBSD, too. How would one go about taking snapshots of a Linux box that is running right on the hardware (nonvirtualized)?