SketchFighter 4000 Alpha redefines "retro" by bringing together the imaginative pen and paper sketches from your youth with a killer resolution independent OpenGL graphics engine. SketchFighter also boasts a unique synthesis of exploration and action game play, which has you alternately blasting cute little lady bugs into oblivion and then puzzling your way through the hand-drawn challenges presented to you.
Project Multi Sketch, a series of 5 custom levels, was released (quite) a while ago. You can get it from here. I was responsible for Area 5.