Commander Keen 2000

From KeenWiki

This was the first ever Commander Keen fangame, produced in 2000, hence the name. Despite this, it is a whole lot better than a lot of ones made later. Most of the graphics are ripped, but there's some new ones too, such as the giant chicken boss near the end.

The game doesn't scroll and has very buggy movement; it is of a reasonable difficulty and set the standards for other fangames.