Doom 3 Cheats

Go to the first level in Co-Op mode and proceed through the level until 'Command Access Junction'. There you can find two Super Turkey Puncher arcade machines made by 'Nabcon'. On the screen is the face of the original Doom Guy and also makes the same faces when you punch the turkeys. First player to 3000 points wins.

Right at the end of the game, just before you fight the final boss, as you are walking through the underground hall with all the fire and brimstone there will be a small space in one of the corners you encounter that you can duck under and there will be a brick with the ID symbol printed on it. Press action on the brick and it will open the wall allowing you to get the PDA of the ID developers thanking you for playing the game.

On the last level before you go through the portal to fight the boss. Their is a stone tablet somewhere in the last room. If you look at it you can see the picture of the original Doom cover.