If you've ever built your own Linux kernel, or installed a hard drive controller card in a Linux box, you know how easy it is to have an "accident" that will prevent the OS from booting completely. Thankfully, Kent Robotti has created an excellent recovery distro called RIP ('R'ecovery 'I's 'P'ossible).
There are a lot of bootable Linux distros out there, but Robotti has a history of making a tight, usable distro called looplinux (a decendant of the first distro I ever installed, doslinux). RIP can boot from a floppy, bootable CD, or even a USB flash drive. It will boot its own Linux OS with a set of basic recovery tools, or let you attempt to boot another system on a different drive. I've had to do both on a few occasions, and without RIP, I would've been totally hosed.