During gameplay type:

IAMACHEATER

The following cheats become available:

C - Add $10,000 every time you hit the key
U - Unlock all items (Unlimited)
S - End Mission

Here are some system options - the IAMACHEATER trigger is NOT needed:

CTRL+I - Toggle Interface On/Off
CTRL+NUM+ - Increase Gamma
CTRL+NUM- - Decrease Gamma
CTRL+[ - Quit Game