Backup Simplicity

Backup Simplicity is a low-cost, set-and-forget backup utility. It makes a daily bootable backup of your OS X system disk to another directly-connected IDE or FireWire disk.

It makes the backup even if you're logged out, or the computer is sleeping. It will save a history of old versions of your files, to the extent of available disk space, and keep a copy of that history on your system disk for safekeeping.