Commander Keen was one of the first ID Software's games. It was a side-scrolling platformer set in a kid, happy theme. The game was released as a shareware, and it became very popular (so popular that ID released six expisodes). The game can be edited using original TED5 editor (used by the makers to create Keen), and a few other fan-created tools and editors.
Commander Keen 4 was one of my first computer games. And it had already the dopefish included, yeah! One of the best platformers ever!