The Commander Keen series is still one of my favorites. Sad thing thing is ID never got around to completing the series because of Doom. Good thing is there is software out now that allows you to mod the game, and people have done that.

The Universe is Toast (mod trilogy)

I have the Commander Keen series because it came in the id Software pack on Steam but I haven't played it yet. I did play Commander Keen in the early-90's but it was probably a demo version; I think I remember my cousin playing it at his house because he had a large speaker connected to his computer and I remember the MIDI being loud.

When I bought my good old gravis gamepad, keen 4 came with it, played the hell out of it since it was one of the few games I had on my pc when I got it. Looking back at it, it wasn't that good... especially keen 1 to 3.