Commander Keen Series

Commander Keen Series cheat codes: 
Mild cheats:
[C]+[T]+[Spacebar] : Free items
[F8]               : Some funny stuff.
[G]+[O]+[D]        : God Mode
[Shift]+[Tab]      : End level

Hard cheats:
Add to the commandline (for series 1-4):
NOWAIT
(This also removes various delays and nagscreens)

Then, in the game, use [F10]+ some letter keys :
[W] Warp to level        [E] Restart
[T] Sprite test          [Y] Mess up sprites
[I] Free items           [S] Toggle slow motion
[D] Record demo          [G] Toggle God mode
[J] Toggle jump cheat    [C] Stats
[V] Add VLB's            [N] Toggle clipping
[M] Memory usage

You may also try adding TEDLEVEL, COMP, NOCOMP.

Secrets:
Episode 4:
In the "Pyramide of moons" get the  yellow gem and
open the door then get all of the 12 worms in one
place and they will turn into
a foot climb the foot and enter the secret level.