I'm a big fan of liveCD tools. They allow me to interact with systems that normally would be either unusable, such as machines with no OS, or difficult to use, such as a PC with an unstable or virus-ridden Windows install. As far as utilities are concerned, I'll take as many as I can get, and apparently Ivan Davidov agrees with me.
ULTILEX, which stands for "Ultimate Linux Experience", is a liveCD with a variety of different projects all crammed on to the same CD. Ivan Davidov was able to fit Slax, Puppy Linux, Tiny Core Linux, System Rescue CD, Parted Magic, FreeDOS, the Ranish Partition Manager, AIDA hardware recognization, Windows password viewing utilites, and hard drive diagnostic and wiping utilities on the same CD (as of Version 4.0). Oh, there's a RAM tester, too. This CD can also act as a PXE server for other machines on your network, if you wanted to try out PXE network booting.
A CD such as this one is invaluable when working on donated computer equipment or used equipment that you are trying to get back in shape, as it allows you to easily test the hardware and run a useful live environment without having to permanently change the hard drive partitions (unless, of course, if you WANT to change the hard drive partitions).
This is another tool that has worked itself into my computer toolkit. Good stuff.