Press ~ during game play to display the console window, then activate the following cheats by entering the corresponding codes:

CHEAT FUNCTION - CHEAT CODE
Spawn indicated item and number - give <1-200>
All weapons - gimme
No clipping mode - noclip
God mode - god
Flight mode - fly
Disable flight and no clipping - walk
All weapons - allweapons
No clipping mode - ghost
Full ammunition - allammo
Toggle invisibility - invisible <0 or 1>
Kill everything - killall
Get everything - loaded
Toggle enemies cannot see - blindenemies <0 or 1>
Toggle AI cannot hear - deafai <0 or 1>
Toggle AI cannot see - blindai <0 or 1>
Toggle enemies cannot hear - deafenemies <0 or 1>
Unknown - unloaded
Unknown - loadspmap
Unknown - supersquad
Unknown - oldmovie
Unknown - lightperiod
Unknown - lightcone
Unknown - getfirstmission
Unknown - pawnanimextra
Unknown - soundocclusion
Unknown - smiteevil
Unknown - avatar
Unknown - summon

Use the following entries with the give code. Note: The weapons are limited to 1 as the number value. Ammunition are limited to 200 as the number value for most weapons:

machete
colt45
machinegun
rifle
bazooka
greasegun
colt45ammo
machinegunammo
rifleammo
greasegunammo
bazookaammo
backpack
grenade
landmine
flamethrower
gasoline

During the first level in Iwo Jima, when you have to clear the beach, find the location where there is a sandbag bunker to the left side of the beach (not the concrete one). If you walk into it, where the right side forms a corner, you will walk right over it and save some frustration.