Doom is famous for being one of the most influential first-person shooters of all time, but would it have worked as a 2D platformer? Given the premise – angry space marine must kill monsters, reach end of level – it probably wouldn't have been as gamechanging, but judging by this fan project, it'd still have been pretty cool.
MiniDOOM is a "very short parody game" based on the first Doom, made as part of a course using GameMaker Studio. While short (it's only an 11MB file, too), it's worth a download just to enjoy the beautiful sprites – the recreation of the Cacodemon is a personal favourite.
Download it over here, or watch the trailer below: