Various Cheats:
During play press the ~ key to get the console then type in one of the following codes

GimmeGimmeGimme - Get all weapons

Invuln - Invincibility

NoClip - No Clipping mode

Spectator - Flying Mode

NoTarget - Monsters ignore you

NextArmor - More armor and health

PrevArmor - Less armor and health

NextMonster - Next Monster

PrevMonster - Previous Monster

CyclePlayerClass - 4 Player classes toggle

RestartLevel - Start level over

Debug 1 - ?????????????

Debug 2 - ?????????????

CameraLock - Lock Camera

Get any inventory item, weapon, or full health:
Start a game (new or saved) and press ~ [that little sqiggly line above the TAB key] to enable the console. type GIVE (you don't need the caps) and press enter. Now a menu will pop up showing you all the items, weapons, or ammo you can get. Now read the format and you should be all set.