Everyone should know about and use Knoppix:

…Knoppix, a Free and Open Source Live Linux CD. Knoppix is a GNU/Linux distribution that boots and runs completely from cd. It includes recent linux software and desktop environments, with programs such as, Abiword, The Gimp, Konqueror, Mozilla, and hundreds of other quality open source programs.

Ever crash your hard drive and need to recover data off of the disk? Ever need to mirror two hard drives? The list goes on and on…