Cheat Effect
impulse 203 Delete Targeted Enemy
r_fullbright 1 (Replace 1 with 0 to turn them back on.) Eliminate Shadows
sv_gravity # (Lower numbers gives less gravity, higher gives more. Normal is 600.) Set Gravity Level
impulse 76 Spawn Marines
thirdperson Third Person View
impulse 101 Instant Weapons, Ammo, and Armor
/god Invincibility
noclip Walk Through Walls
impulse 102 Spawn Gibs
notarget Invisibility
give ammo_357 Ammo for the 357 magnum.
give ammo_9mmbox Ammo for the glock pistol.
give ammo_9mmclip 17 ammo for the glock pistol.
give ammo_crossbow Crossbow ammo.
give ammo_mp5grenades Mp5 grenade ammo.
give item_healthkit Gives you some health.
give weapon_9mmhandgun Gives you the glock pistol.
give weapon_357 Gives you the 357 Magnum.
give weapon_crossbow Gives you the crossbow.
give weapon_crowbar Gives you the crowbar.
give weapon_handgrenade Gives you some hand grenades.
give weapon_mp5 Gives you the MP5.
give weapon_hornetgun Gives you the hornet gun.
give weapon_gauss Gives you the guass rifle.
give weapon_rpg Gives you the rocket launcher.
give weapon_shotgun Gives you the shotgun.
give weapon_snark Gives you some snarks.
give weapon_tripmine Gives you some laser tripmines.
give item_suit Gives you the HEV suit.
chase_active 1 Third Person
give ammo_buckshot Shotgun ammo.

Impulse Codes
After activating the conlse, press tide (~) and enter the following.
Cheat Effect
impulse 101 Get all weapons and armor. Enter again for more amo and/or armor.
impulse 102 Spawn gibs.
impulse 76 Creates a human grunt. Be careful, this can crash the game.
impulse 103 While looking at amonster, displays stats.
impulse 104 Lists global entities
impulse 105 Enemies can't hear you
impulse 106 While looking at an object, displays model/sprite stats.
impulse 107 While looking at an object, displays the texture name.
impulse 109 Control onscreen monsters. This code may not work in versions of the game.
impulse 195 Displays AI node information
impusle 202 Creates blood infront of you, like spray paint.
impulse 203 Deletes target NPC or monster from the game.

Items from the console
Go to the start menu, then go to all programs, then go to accessesories, then click on command prompt. Select you Half-Life directory and type hl.exe -console then Half-Life should open. Start a new game then press the ` (tilde) key then type in these codes:
Cheat
give ammo_357 357 magnum ammo
give ammo_9mmclip Pistol clip
give item_suit HEV Suit
give item_healthkit Health
give weapon_egon Gives you the egon!
give weapon_gauss Gives you a Guass Rifle
give weapon_mp5 Gives you a MP5
give weapon_shotgun Gives you a Shotgun
give weapon_crossbow Gives you a Crossbow
give weapon_9mmhandgun Gives you a Pistol
give weapon_handgrenade Gives you some Hand Grenades
give weapon_rpg Gives you a Rocket Launcher
give weapon_crowbar Gives you a Crowbar
give ammo_mp5grenades Gives you some MP5 Grenades
give weapon_satchel Gives you a Satchel Charge
give ammo_rpgclip Gives you a Rocket
give weapon_hornetgun Gives you a Hornet Gun
give ammo_glockclip Gives you a Glock pistol clip
give weapon_python Gives you a 357 Magnum
give weapon_tripmine Gives you a Laser Trip Mine
ammo_crossbow Gives you Crossbow ammo
give item_longjump Gives you the Long Jump Module

give weapon_SLAM Gives you the slam weapon