"Blinkensisters - Hunt for The Lost Pixels" is a Jump'n'Run game for nearly all recent operating systems. It's fun to play and you can make your own levels with a simple texteditor. Also, you can download add-ons from within the game engine.
A multiplayer deathmatch game in the spirit of the bomberman series, built with user modification in mind. Levels and weapons can be easily created with just a simple texteditor and the Gimp. Levels are tile-based, with animation and layering.