As others have already mentioned, IDKFA for Doom and didn't it work in Doom 2 as well?
I only ask because when ID Software released Heretic, a swords and sorcery style Doom game, I remember typing in IDKFA, expecting the same results. It was the Doom engine after all. Except typing in the classic code kills you instantly!
Another isn't strictly a cheat code. On the old 8bit computers the UK, you could enter some programming code to change the memory before loading in a game. (This is over simplified.)
Because of the altered memory, it would effect the game in some way. E.g. infinity health or lives.
I found a listing in a magazine which you could type in to help with the ZX Spectrum version of Batman: The Movie.
In this platform game, with the cheat, when a bad guy tried to climb a ladder, they would freeze.
This was very helpful and worked wonderfully all the way through the game. That was until you reached the very end whereupon the Joker was on a rope ladder attached to a helicopter.
The code interfered with the end of game boss in a way it wasn't expecting and the game would crash.
Thanks.