Y.A.G.A.C.
Y.A.G.A.C is a free open-source platformer game
...It's a story-less game, heavily gameplay based, where you play a ninja trapped in dungeons filled with traps and robots. As you collect colored balls hidden around the level, so the platforms of that color become solid and you can then reach new places inside the level. A very challenging game. Excellent and very smooth engine, the game's strongest point.