Press the T key to activate cheat mode, then enter the following codes:

MPGOD - God mode
MPHEALTH - Full health
MPARMOR - Full armor
MPAMMO - Reload weapons
MPKFA - All weapons and armor
MPMIMIMI - All weapons
MPTEARS - Unlimited ammo
MPBOYISUCK - Kill all enemies
MPONFOOT - On foot mode
MPMECH - Mech mode
MPVEHICLE - Vehicle mode
MPTELEPORT - Teleport to start
MPTRIGGERS - Event triggers visible as colored cubes
MPCLIP - Clipping mode on/off
MPPOS - Show coordinates
MPCAMERA - Camera settings
MPLIGHTSCAPE - Brightness settings
MPBIGGUNS - Big weapons