From http://www.halflife.net

The cheats

god
The standard cheat that will make you invunerable to any damage. You can't even get squished with this cheat on.

noclip
Allows you to walk through walls. This cheat works a little different in Half-Life than in other games. You can still
get hurt from enemy fire, you can pick up items, and you'll will go to the next level where you would normally do it.
Generally you could say that this cheat just allows you to move a bit more around, it doesn't make you more airy.

notarget
This cool cheat will make you invisible to enemies, meaning they won't attack you, no matter what you do to them.
This cheat is great for taking screenshots.

map xxxxxx
Takes you to a specified map. The map names are a bit complexed in Half-Life, but here's a list of them.

Hazard Course:
t0a0 ; t0a0a ; t0a0b ; t0a0b1 ; t0a0b2 ; t0a0c ; t0a0d

Black Mesa Inbound:
c0a0 ; c0a0a ; c0a0b ; c0a0c ; c0a0d ; c0a0e

Anomalous Materials:
c1a0 ; c1a0a ; c1a0b ; c1a0c ; c1a0d ; c1a0e

Unforeseen Consequenses:
c1a1 ; c1a1a ; c1a1b ; c1a1c ; c1a1d ; c1a1f

Office Complex:
c1a2 ; c1a2a ; c1a2b ; c1a2c ; c1a2d

'We've Got Hostiles':
c1a3 ; c1a3a ; c1a3b ; c1a3c ; c1a3d

Blast Pit:
c1a4 ; c1a4b ; c1a4d ; c1a4e ; c1a4f ; c1a4g ; c1a4i ; c1a4j ; c1a4k

Power Up
c2a1 ; c2a1a ; c2a1b

On A Rail:
c2a2 ; c2a2a ; c2a2b1 ; c2a2b2 ; c2a2c ; c2a2d ; c2a2e ; c2a2f ; c2a2g ; c2a2h

Apprehension:
c2a3 ; c2a3a ; c2a3b ; c2a3c ; c2a3d ; c2a3e (where you get ambushed)

Residue Processing:
c2a4 ; c2a4a ; c2a4b ; c2a4c

Questionable Ethics:
c2a4d ; c2a4e ; c2a4f ; c2a4g

Surface Tension:
c2a5 ; c2a5a ; c2a5b ; c2a5c ; c2a5d ; c2a5e ; c2a5f ; c2a5g ; c2a5w ; c2a5x

"Forget About Freeman!"
c3a1 ; c3a1a ; c3a1b

Lambda Core:
c3a2 ; c3a2a ; c3a2b ; c3a2c ; c3a2d ; c3a2e ; c3a2f

Xen:
c4a1

Gonarch's Lair:
c4a2 ; c4a2a ; c4a2b

Interloper:
c4a1a ; c4a1b ; c4a1c ; c4a1d ; c4a1e; c4a1f

Nihilanth:
c4a3

The Ending Sequence:
c5a1

give xxxxxx
This allows you to get a weapon, ammo pack or item of your choice. Choose amongst the following objects:

Items:
item_suit ; item_longjump ; item_healthkit ; item_battery

Weapons:
weapon_crowbar ; weapon_glock ; weapon_357 ; weapon_mp5 ; weapon_shotgun ; weapon_crossbow ;
weapon_rpg ; weapon_gauss ; weapon_egon ; weapon_hornetgun ; weapon_handgrenade ; weapon_satchel ;
weapon_tripmine ; weapon_snark

Ammo:
ammo_glockclip ; ammo_buckshot ; ammo_mp5clip ; ammo_mp5grenades ; ammo_357 ; ammo_crossbow ;
ammo_egonclip ; ammo_rpgclip 

Monster Spawning

This is basically a give cheat, but it has limits. It allows you to spawn monsters, including Security guards, Military
Weapons, etc., at a map, but the map must contain the monster, or Half-Life will exit to Windows with the error "
Bad Entity in IndexOfEdict()". To check which entities a map contains, type "entities" in the console, and keep
scrolling up (PageUp/PageDown) to see which models are loaded for the map. Personally I can never get higher up
than about number 40, this is probably a limit in the scrolling lenght in the game. I'm using 800x600 resolution,
higher res might help this. When you've found out what you can spawn, there a few things you have to keep in
mind:

     Monsters will spawn excactly where you are, so use the noclip cheat, or you'll get stuck inside it.
     Monsters will not be able to move if their bounding box is trapped inside a brush, so float a bit above the
     ground, and then spawn it.
     If you don't want monsters to attack you (good for taking screenshots), you can use the notarget cheat. 

Monsters you can spawn, those with XXX in the brackets have very limited and/or useless, maybe doubtful effect:

alien_slave (Slave), alien_grunt (Alien Grunt), apache (Apache Chopper), barnacle (Barnacle), barney (Secutiry
Guard), barney_dead (Dead Secutiry Guard), bigmomma (Gonarch), cockroach (Cockroach), furniture (XXX),
flyer_flock (flock of birds as seen on c5a1?), gargantua (Garg), generic (XXX), gman (Guy In The Blue Suit),
grunt_repel (Grunt?), headcrab (Headcrab), hevsuit_dead (Dead guy in HEV-suit), hgrunt_dead (Dead Grunt),
human_assassin (Assassin), human_grunt (Grunt), houndeye (Houndeye), scientist (Scientist), scientist_dead
(Dead Scientist), sentry (Light Mahcinegun), miniturret (Light Ceiling-Gun), turret (Heavy Ceiling-Gun), ichthyosaur
(Ichthyosaur), leech (Leech), tentacle (Tentacle), zombie (Zombie), hgrunt (Grunt), nihilanth (Nihilanth), osprey
(V-22 Osprey Plane)

Bear in mind that some monsters will act very weird when spawned in places that are not their natural habit. For
example, an Itchyosaur won't start swimming around in the air, as will Nihilanth's AI not work properly elsewhere
than on c4a3.

Greets to I.am/Half-Life for most of this info.

impulse 101
This neat cheat will give you one of each item in the game, except a health pack and a battery. Type it twelve
times and you'll have full ammo. To make cheating easier, you can bind a key to it, like this:

bind c "impulse 101"

Put the above lines in your autoexec.cfg (create a new text-file with notepad and save it in your Half-Life\Valve dir if
it isn't already there), and replace the "c" with the button you want. You can now get all weapons and lots of ammo
with the press of a button. Press it about 11 times to get fully charged. 

Before you can use any of these cheats, you have to enable cheats in the console. Run Half-Life with the -console switch to enable the console if you don't have it already.

To enable cheats, the server variable "sv_cheats" has to be set to 1. The change won't take effect until a level change has taken place, so:

1) Type "sv_cheats 1" (minus quotes) in the console. If the game is multiplayer, you must be running the server to do this or use rcon.
2) In a single player game, either press F6 to save the game and then F7 to load the game or go through a level change. In a multiplayer game, use the changelevel command to change level or wait for a level change to happen as normal.

Once sv_cheats is enabled, you'll be able to use all the cheats listed above.

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