SuperTux is a free, open-source video game very similar to the old "Super Mario Bros." for the Nintendo. Instead of leading an Italian guy through a surrealistic landscape, you lead a penguin across the landscape of Antarctica.
The left and right arrows on the keyboard control Tux left and right, the up button makes him jump, and down makes him crouch. The left "Ctrl" button makes him run or shoot fireballs if you have that capability.
SuperTux runs on Windows, Linux and Mac OSX.