At the Main menu press Up, Up, Down, Down, Y, Down, Up, Y to unlock the Cheat Screen then select the 'Add Cheat' option before entering any of the following Case-Sensitive Cheats to unlock the corresponding effect. These codes are case sensitive.
Unlocks ALL Levels and Emergencies: KEYTOTHECITY
Unlock Long Range Stungun: FRYINGTIME
Unlock Mini gun (replaces the Pistol): MINIFUN
Bullets Set Fire to Enemies (similar to one hit kills): BURNINGBULLET
Thermal Breather MK2 (for visibilty in smoke): ISEEYOU
Unlock MK3 Assault Rifle with Unlimited Grenades: ULTIMATEPOWER