Tux Paint is a simple and entertaining drawing program geared towards young children. It has a simple interface, sound effects, and a cartoon character (Tux, the Linux penguin). Along with drawing brush strokes, lines and shapes, you can also enter text and place "rubber stamp" (or "sticker") images on the picture. Tux Paint is extensible, and could be useful in an educational environment (such as a grammar, elementary, or grade school). It's portable across numerous platforms, and runs well even on slower systems like the Pentium 133MHz.
Lux is a game of strategy and domination inspired by the board game Risk. Control your armies to conquer and hold strategic countries on the map. The object of the game is to take over the entire world. In addition to the classic Risk map, Lux has a variety of different maps available and features a map editor to create new maps. The map files are in a documented XML format. Lux comes with computer AI players ranging from easy to hard. There is also an SDK for creating robots and an Internet game tracker.
GearHead is a mecha roguelike roleplaying game. Set a century and a half after nuclear war, you can explore a world where various factions compete to determine the future of the human race. Major features include random plot generation, a detailed character system, and over two hundred customizable mecha designs.