Cheat: Skip Level
Call up the PDA and select the Objectives tab. Hold LEFT TRIGGER and enter B, A, X, Y. When you leave the PDA screen, your space marine is zapped to the next map with what ever you have at the moment.

Cheats: Classic Doom
These are the Ultimate DOOM and DOOM 2 cheats which are only included with the Limited Edition Doom 3 game.

All Keys, All Weapons, and Full Ammo: hold down LEFT TRIGGER and press A,B,A,B
Berserk Mode: hold LEFT TRIGGER and press X,X,X,Y.
End Level: hold down LEFT TRIGGER and press A,A,B,B.
Chainsaw: hold down LEFT TRIGGER and press B,B,A,A.
Weapons and Ammo: hold down LEFT TRIGGER and press B,B,B,B.
GOD Mode: hold down LEFT TRIGGER and press A,A,A,A.
Invisibility: hold down LEFT TRIGGER and press X,X,Y,Y.
Light Amplification Goggles: hold down LEFT TRIGGER and press Y,Y,Y,X.
Radiation Suit: hold down LEFT TRIGGER and press X,Y,Y,Y.
Show All Map: hold down LEFT TRIGGER and press Y,Y,Y,Y.
Temporary Invincibility: hold down LEFT TRIGGER and press X,X,X,X.