Pytonis Universalis (name is still WIP) is a simple shoot 'em up. It is written completely in Python and uses Pygame so the code is as accessible as possible for others to inspect.
The idea should be easy enough for anyone to understand: try to avoid running into flying saucers and their laser beams by shooting them down before they shoot you down. Every time you destroy a flying saucer you get points. Points are nice, you want them.