Press Left Shift and F11, then type one of the following.
Code - Effect
behind the enemy horse - Display all briefings.
clint - Win Level.
epitaph - Show Victory Conditions.
fidel castro - View old dialog boxes.
god - don't die.
hollow man - Invisibility.
jackal - Give Ammo.
medic - View Hints.
powerman - New Weapon.
schneider - Lose Level.
show me all - Show All Objects.
supersonic - Toggle sound zone display.
timeless - Stop time.
whats my destiny - Toggle short briefings.
zeus - Kill enemy with lightning bolt.
Invincible, Next level
During the game press left shift and f11 at the same time. You will now get a cheat menu. Type HOLLOW MAN (enter) to be invincible. Hit enter to get back to the game.
To win the level and get to the next level, you go to the cheat menu (left shift and F11) and you type CLINT (enter). To get back to the game hit enter.