While playing, hold [CTRL] + [SHIFT] + [O] + [E] and then press [H] to activate Developer mode. You can then press any of the following cheat keys.

Code	Effect
Key 8	Add 1000 EXP
Key 6	Give Explosives
Key 5	Give Gas Mask
Key 9	Give Morphine
Key 2	Give Next Handgun
Key 1	Give Next Rifle
Key G	God Mode
Key 7	Intended Upgrade Attributes
Key N	Maximum Ammo for Current Weapon
Key M	Maximum Armor
Key X	Toggle Fly Mode
Key V	Toggle Visibility