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.