Slax: Your Pocket Operating System
Slax is a fast, small, portable Linux distribution taking a modular approach that gives you the ability to easily add on your favorite software. Just download a module with the software, copy it to Slax -- no installing, no configuring.
See it in action:
TheMacWizard posted the following video on YouTube demonstrating Slax on his Macbook (running Leopard 10.5.4).
PenDriveLinux also posted a demo of Slax on his Windows machine.