The Four Golden Crystals
The Four Golden Crystals, also known as Commander Keen in the Cartoon Realm, is a Keen 1 mod produced by Unfleexable. Although all levels are there and it's possible to complete the game, the creator said he doesn't consider it as finished. The mod suffers from its poor level editing and isn't much fun to play.
It also contains a number of characters from the modder's comics and has odd, cartoonish graphics. It is one of the earlier mods and is reminiscent of Shadow Keen 1 and 2 in that its creator has not yet got the hang of 16-color graphics.
Spoiler Warning! This shows the whole levels.