Do you remember the OS X game SketchFighter, the shooter made from pencil drawings? Sketch Nation Shooter is certainly very similar, letting you draw your own levels, players and even enemies. You pretty much design the game yourself!
It's $1.19, and from what I can tell sounds worth every cent. It's compatible with iPhones, iPod Touches and the iPad, and if you're not feeling too creative then you can download other players' attempts too. The game is apparently available on Facebook too, meaning you can send your lovingly hand-crafted game to Facebook players as well. [iTunes]