Use notepad to edit the "bia.ini" file in the "system" folder in the game directory. Look for the "[Engine.GameInfo]" heading, then enter the line "bCheatsEnabled=True" below it. Then, press ~ during game play to display the console window.

Effect - Cheat Code

All weapons - allweapons
Disable flight and no clipping mode - walk
Extra ammunition - allammo
Flight mode - fly
Get all items - loaded
God mode - god
Kill everything - killall
No clipping mode - ghost
Old movie mode - oldmovie
Remove all items - unloaded
Select map - loadspmap [map name]
Spawn indicated item - summon [item name]
Toggle blind enemies - blindenemies [0 or1]
Toggle blind AI - blindai [0 or 1]
Toggle deaf AI - deafai [0 or 1]
Toggle deaf enemies - deafenemies [0 or 1]
Toggle invisibility - invisible [0 or 1]