Go to any opening screen or menu, type "iamcheater" or "iwillcheat" to enable cheat mode. Then while playing the game or at any menu screen, enter one of the following:
Toggle invincibility: cantdie
Invincible to gunfire: nohits
Full health: goodhealth
All weapons with full ammo: allloot
All items: allitems
Open all doors: openalldoor
Kill all opponents: killthemall
Lose current mission: gamefail
Win current mission: gamedone
Enemy view from the front: enemyf
Enemy view from the back: enemyb
Show coordinates: playercoords
Big head mode: funnyhead
Wire frame mode: debugdrawwire
Load saved game of current mission: quickload
View ending sequence: showtheend