Pocket Size OS

Slax is a modern, portable, small and fast Linux operating system with a modular approach and outstanding design.

http://www.slax.org/

MenuetOS is an Operating System in development for the PC written entirely in 32/64 bit assembly language

http://www.menuetos.net/

UserModeLinux
User-Mode Linux gives you a virtual machine that may have more hardware and software virtual resources than your actual, physical computer.

http://user-mode-linux.sourceforge.net/