This is an open source NES platformer engine that powers Nova the Squirrel (Design details here). It's named because it's similar in purpose to the President engine.

Right now things need to be worked to be generic and be a better starting-off point. This engine will provide something much more legal to play with than hacking a game in the Mario series. In addition, unlike with Super Mario World, all the tools as well as the game itself are completely open source, and GPL licensed.