LiveCD Linux distros

• Knoppix — great general-purpose tool, with the most drivers available
• BackTrack — gray hat / black hat, especially wifi cracking
• DamnSmallLinux — for tight spaces

DOS apps

• Ghost (Symantec) — Clone/backup drive partitions.
• Partition Magic — resize partitions (Parted is an open source version that's nearly as good)
• SpinRite — okay, I've never actually used it successfully, but it's said to be good, supposedly
• PC Check (Eurosoft) — good for laptop hardware debugging [1] [2]

Floppies

• memtest86+
• semi-useful, or niche/infrequently used
  • Bootable CD Loader — fixes most BIOS problems with booting CDs