HT Games
God Mode
Enabling "God Mode" makes your character impervious to damages from enemy weapons. While you still incur a death from falling off a ledge that leads to a bottomless pit or other environmental automatic kills, you are completely invulnerable to enemy weapons fire. To enable this cheat, type "/god" into the console while in an active game.
No Clip
The source engine "clips" your character into the environment by default, to ensure you are interacting with the surroundings properly. This clipping effect is what ensures you can not walk through walls or other obstacles, and keeps you tied to the ground. Typing "noclip" into the console allows you to move up and down freely, as though unaffected by gravity, and also allows you to pass through walls and even outside of the level geometry into the sky-box above.
No Target
If being completely invincible doesn't appeal to you very much, or you would like to explore without being harassed by enemy troops, you can type "notarget" into the console to render yourself invisible. You still incur both environmental and weapons damage, if someone happens to fire at you blindly, but can otherwise move about undisturbed without fear of being discovered.
All Weapons and Armor:
Typing "impulse 101" into the console outfits you with a full compliment of weapons and armor. It does not give you unlimited ammunition, but entering the code again resupplies your stock of ammo and fully repairs any damage that has been done to your armor. This code is sure to keep you fighting through even the toughest of sieges.