http://www.knopper.net/knoppix/index-en.html
Perhaps the most famous of the Linux LiveCD invasion. I've used versions for i86 and PPC.
Testimonal from IMBJR: Recently, my PC lost the ability to boot to Windows. Fortunately, I had a Knoppix boot disk and was able to burn a disk with all my as-yet-unarchived work on it.