Puppy Linux is a lightweight distribution built to run in memory and therefore the overall footprint is very small.
Puppy is designed to run from a USB drive and not for installation on a hard drive.
There are a number of Puppy derivatives available including MacPup and Simplicity.
Puppy Arcade is designed for fun. It includes emulators for every games console imaginable as well as ROM loading software and joystick calibration.