Commander Genius vs Natural Selection (Series)

Commander Genius is an open source clone of the classic DOS game 'Commander Keen'. It features retro-style platforming gameplay with puzzles to solve across multiple episodes. As the hero 'Billy Blaze', players run, jump, and shoot their way through levels to stop the evil alien threat.

Natural Selection is a mod for the first-person shooter Half-Life that eventually turned into a full stand-alone game. It features complex real-time strategy gameplay with two opposing alien races fighting for control of territory and resources.