To enable the Cheat mode, hit the Backspace key, type !when all else fails and hit Enter. Now, hit Backspace again and type one of the following:

!BEAUTIFUL NIKITA	Gives you infinite use of the snipers rifle.
!DOCTOR DOCTOR	Restores you to full health.
!GERONIMO!	Calls in an air strike.
!GOD OF GAMBLERS	Gives you a random item.
!I GIVE UP	Restarts the level.
!I HAVE A ROCK	Gives you infinite grenades.
!JUMPJETS	Lets you fly.
!NIGHT OF THE WALKING DEAD	Turns units into zombies.
!NINJA ARTS	Puts you in stealth mode.
!PAPER DOLLS	Calls in paratroopers.
!PATTON'S SPEACH	Promotes highlighted men
!ROACH SPRAY	Gives you infinite Aerosol spray.
!RUBBER CEMENT	Gives you a Medkit.
!SANTINI	Puts you in god mode.
!SPIDEY SENSES TINGLING	Reveals the entire map.
!SURPRISE PARTY	Increase the number of enemies.
!VENI VIDI VINCI	Wins the mission.
!VILLAGE POEPLE	Gives you infinite flamethrower.