Doom 2 [DeathMatch]
There seems to be something strange with the IDDT cheat
(the "map-mode"). It works on the Nightmare level, where
as the others don't.
But the real interesting thing is: Hold [Alt] while typing
IDDT 3 times, on the map screen in deathmatch mode enables
you to see all monsters and your human enemies!
This may not work on all versions (surely not on version 1.9
and above) but we tought it was too important not to mention.