Cheat Mode:
Enter +set console 1 as a command line parameter to enable cheat mode. Here's an easy way of doing this:

Create a Shortcut to the game on your Desktop. Right click on it, go to properties. You will see something like:

"C:\Games\SoF\SoF.exe"

Now type +set console after it:

"C:\Games\SoF\SoF.exe" +set console 1

Once in the game press the @ or ~ to access the console and type \cheats 1. Now you may enter the following cheats.

HERETIC         - God mode
ELBOW           - Weapons 1-5
BIGELBOW        - Weapons 6-10
MATRIX #        - Timescale (replace # with 1-10) eg Slow, Fast motion.
PHANTOM         - Noclip
NINJA           - Notarget (can't be seen by enemys)
MAP (XXX)       - Level select (where XXX is the name of the level, see below)
DEFAULTWEAPONS  - Gives default weapons
UPDATINVFINAL   - Gives more ammo
KILLALLMONSTERS - Kills all enemies
KILL            - Suicide
GIMME(XXX)      - Get item XXX. Replace XXX with one of the following:

item_weapon_assault_rifle
item_weapon_machinegun
item_weapon_pistol
item_weapon_pistol2
item_weapon_rocketlauncher
item_weapon_sniper_rifle
item_weapon_shotgun
item_ammo_battery
item_ammo_slug
item_ammo_gas
item_ammo_auto
item_ammo_rocket
item_ammo_shotgun
item_ammo_pistol
item_ammo_pistol2
item_equip_medkit
item_equip_armor
item_equip_c4
item_equip_flashpack
item_equip_grenade
item_equip_light_goggles
m_nyc_ebum
m_nyc_mskinhead1
m_nyc_politician
m_nyc_mpunk 

Level names to use with Map cheat:

TSR1 
TSR2 (just a cinematic) 
TRN1 
ARM1 
KOS1 
KOS2 
KOS3 
SIB1 
SIB2 
SIB3 
IRQ1A 
IRQ2A 
IRQ3A 
ARM2 
NYC1 
NYC2 
NYC3 
SUD1 
SUD2 
SUD3 
ARM3 
JPN1 
JPN2 
JPN3 
IRQ1B 
IRQ2B 
IRQ3B 
GER1 
GER2 
GER3 
GER4 
 

Thanks to Revolution reader Mikey, Grudgebringer, Tonic and neon!