Various Cheats:
During gameplay press ~ to get the console then put in a code below

give all - Get all items

give life - Get Life

pause - Pauses game

toggle g_p_god - Invincibility

give jetpack - Get jetpack

give ammo - Get ammo

give nuke - 10 Nukes

give secret - Secret marked as found

give keys - Get keys

Give forcefield - Get forcefield

toggle r_stats - ????

camera player - ????

kill - ????

toggle r_timings - ????

toggle g_map_info - ????

toggle g_debug - ????

camera camera - ????

Other Cheat Method:
Press ~ to bring up the console then type in exec cheats.cfg and press ENTER to activate cheats

Now press a letter below

G - Get Everything
H - Max Ammo
J - Get Jetpack
F - Get Forcefield
L - 1 Extra Life
K - Self-Destruct
I - Invinicibility
P - Pause