I had already posted about SystemRescueCd (website here)
Just ran across Ultimate Boot CD
Looks pretty handy - another tool for the box. UBCD is more DOS oriented with a bunch of tools for dealiing with older machines.
SRCD is a bootable Linux distribution with disk tools.
Cool thing with UBCD is that you can run Floppy disk utilities without have a floppy disk in the machine under test.