Press [T] to open the command-line, then type:
Code - Result:
god - God Mode
armor - Full Armor
health - Full Health
poltergeist - Invisibility
maphole - Skip Level
skillz - Get Skillpoints (Level 3-4)
pos - Show Position
build - Show Version
baddaboom - Unknown Effect
guns - All Guns
ammo - Full Ammo
kfa - All Weapons, Ammo, Health, Armor
mods - Scopes, Silencer & Camera Zoom
build - Show Version
baddaboom - ???
rosebud - ???


Unlock All Chapters
To open all chapters of the game, run REGEDIT and find the key:

[HKEY_LOCAL_MACHINE\SOFTWARE\Monolith Productions\No One Lives Forever 2\1.0]

Change the value of "EndGame" to 1.