To use codes, you have to run the game with the console enabled. Launch Half-Life in developer mode by going to your Windows Start menu, selecting "Run", and typing
C:\Sierra\Half-Life\hl.exe -dev -toconsole
Now load the last quick saved game (F6), then press the ~ key to pull down the console. Type in sv_cheats 1 and press Enter. Cheats are now enabled, but you need to die or reload your game before you can use them.

Now during play, press the ~ key and type in the following codes:

/god 		- God mode on/off
/notarget	- Enemies ignore you
/impulse 101- All weapons and ammo
/impulse 102- Spawns giblets and blood 
/impulse 103- Shows monster stats while looking at monster
/impulse 104- List global entities 
/impulse 105- Silent Player
/impulse 106- Model/sprite stats, while looking at target 
/impulse 107- Texture name, while looking at object
/impulse 195- AI mode on
/impulse 203- Delete monster or NPC from world 
/thirdperson- Third person view  
/firstperson- Return to first person view    
/noclip 	- No Clipping (Walk Through Walls, Etc.)
/map C#A# 	- Go to any map (enter numbers for #: C1A1, C1A2, Etc.)
developer 1 - Developer mode on   
developer 2 - Verbose developer mode    
developer 0 - Developer mode off 
/give XXXX 	- Gives any item. In place of "XXXX", enter one of the following:


item_airtank 
item_antidote 
item_battery 
item_healthkit 
item_longjump 
item_security 
item_sodacan 
item_suit
ammo_357 
ammo_9mmar 
ammo_9mmbox 
ammo_9mmclip 
ammo_argrenades 
ammo_buckshot 
ammo_crossbow 
ammo_egonclip 
ammo_gaussclip 
ammo_glockclip 
ammo_mp5clip 
ammo_mp5grenades 
ammo_rpgclip
weapon_357 
weapon_9mmar 
weapon_9mmhandgun 
weapon_crossbow 
weapon_crowbar 
weapon_egon 
weapon_gauss 
weapon_glock 
weapon_handgrenade 
weapon_hornetgun 
weapon_mp5 
weapon_python 
weapon_rpg 
weapon_satchel 
weapon_shotgun 
weapon_snark 
weapon_tripmine

Thanks to Revolution readers Rickard Liljedahl, wolvie21, James Hernandez, Vince, Freddy Lansky, Bjorn Lindqvist, Simon Eliasson, Andreas Fougner, Coal Flower, Niclas Hedlund, SeaFox, ROBO, XL83, hXc, and Jared Leonard!