Doom

Cheat mode:
Enter one of the following codes during game play to activate the corresponding cheat function.

Effect

Code

Invincibility (God Mode)

iddqd

All weapons, keys, ammo, and 200% armor

idkfa

Berserker strength

idbeholds

Temporary invincibility

idbeholdv 

Temporary invisibility

idbeholdi

Full Automap

idbeholda

Anti-radiation suit

idbeholdr

Light amplification visor

idbeholdl

Walk through walls

idspispopd

Chainsaw

idchoppers

Level select

idclev<episode number><map number>

Toggle full map; full map and objects; normal map

iddt

Display coordinates and heading

idmypos

Change music to indicated level

idmus<level number>

Hint: Using sounds to find monsters:
Listen for weird sounds, like somebody breathing very hard during the game. Those sounds are the brown monsters that throw fireballs. When the sounds stop, that means you have killed them all. However, there still may be normal enemies with guns and shotguns or pig monsters.

Hint: Nightmare mode:
Cheat codes cannot be used in nightmare mode. It is just like the regular game, except the monsters are stronger and harder to kill. Enemies will keep appearing non-stop on each level -- you have to try and get to the end before more appear.

Glitch: Death on stairs:
Kill an enemy while he is on the side of the stairs. The enemy will die, but will be floating on the side of the stairs. If you kill an enemy at the top of the steps, the enemy should dead but fall down the steps.

Glitch: Living death:
Allow an enemy to kill you. Once you have a bloody face and you are on the ground, enable the "God mode" code. Your character will come alive again, but will not be able to change guns, move, or shoot.