Doom 3

After you get you shotgun and reach the security area, go in side and unlock the door. After the door is unlocked, on the same screen, look at the botom of the screen and click on one of the 2 choices. One should bring up the lockers, showing one is locked and the other open. Click on the one that says locked. Now head out into the elavator room and kill the imp that jumps down. Look at the lockers now, the other one is now open, allowing you to have the machine gun and more shotgun ammo. (Note: After you grab all the goodies in the locker, an imp will come out of the door you just came out of to get the goodies.)

Each of these PDAs are found throughout Doom 3:

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.

These codes can be entered any time while playing the game.
;All codes can be toggled on/off, except the All-Weapons codes, and the Computer Map code, which cannot be toggled off

Doom3 Collector's edition and Doom3 Resurraction of Evil for xbox both have Ultimate doom and Doom2 included with it.

There are two exclusive levels created just for these 2 games..

Ultimate doom secret level - Sewers Episode 1 mission 10
This level can be accessed by going through the first episode Knee deep in the dead in the very first level.. The exit is located in the outside area with the blue armor and go against the wall using the action button and you shall exit the level into the new level..

Doom 2 New Secret Level - Betray level 33

To access this secret level, play level 2 entryway, go to the area where you find the red key card, when you get it, a room with sargents and imps will open, go kill them all and go into that little room with the imps which is on the right side, push the action while you walk around agains the wall and you shall exit the level.