Commander Keen 1: Marooned on Mars

While his parents are out, eight-year-old genius Billy Blaze dons his brother's football helmet, climbs into the homemade Bean-with-Bacon Megarocket and becomes Commander Keen, defender of Earth. Stranded on Mars after the Vorticons steal vital parts of his ship, Keen must recover them from the Martian cities to get home.

Released by Apogee on 14 December 1990, Marooned on Mars is the shareware first episode of Invasion of the Vorticons. Its smooth side-scrolling — pioneered by id Software on the PC — led directly to the company's later work on Wolfenstein 3D and Doom.