Half-Life

To use codes, you have to run the game with the console enabled.
Launch Half-Life in developer mode by going to your Windows Startmenu,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
/noclip - No Clipping (Walk Through Walls, Etc.)
/map C#A# - Go to any map (enter numbers for #: C1A1, C1A2, Etc.)
/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
(Information from Cheat Machine)