While playing, press Talk (the default key is "T") then type any of the following codes and press (Enter):

Result - Code:

Full armor - armor
Full health - health
Position mode - pos
All weapons and unlimited ammo - tears
Ghost mode - poltergeist
Increase health and reflexes - gear
Level skip - maphole
Weapons, full ammo, armor, health - kfa
Spawn indicated weapon - gimmegun (name)
Spawn indicated ammunition - gimmeammo (name)
Display build version - build
Invulnerability - god
Invisibility - notarget
Full ammunition - ammo
All weapons - guns