Cheat mode:
First load the game with the command line: hl.exe -dev -toconsole. Now, push ~ if the orange text on the screen does not disappear. While playing the game, push ~ to display the text prompt and type in sv_cheats 1. Push ~ and submit one of the console commands below to enable the corresponding cheat function.

What the Code does	Code
God mode	/god
Adjust gravity	/sv_gravity <-999 - 999999>
All weapons and ammo	/impulse 101
Developer mode on	/developer 1
Verbose developer mode	/developer 2
Developer mode off	/developer 0
Suicide	/kill
Map select	/map <map name>
No clipping mode	/noclip
Obtain item	/give <item name>
Opponents ignore character	/notarget
Third person view	/thirdperson
Return to first person view	/firstperson
Auto-reload enabled	/+reload
Auto-reload disabled	/-reload
Speak word	/speak <word>
Speak sentence	/speak <word.word2.word3>
See things brightly without flashlight	/lambert -1.0001
Toggle light amplifier	/r_fullbright <0,1>
Toggle game speed	/host_framerate <0,1>
HEV suit, weapons, and ammo	/impulse 101
Spawns gibs and blood	/impulse 102
Monster stats, while looking at monster	/impulse 103
List global entities	/impulse 104
Silent player	/impulse 105
Model/sprite stats, while looking at target	/impulse 106
Texture name, while looking at object	/impulse 107
Control onscreen monsters	/impulse 109
AI node information	/impulse 195
AI node information	/impulse 196
AI node information	/impulse 197
AI node information	/impulse 199
Delete monster or NPC from world	/impulse 203

Map name list:

Single player
c0a0
c0a0a
c0a0b
c0a0c
c0a0d
c0a0e
c1a0
c1a0a
c1a0b
c1a0c
c1a0d
c1a0e
c1a1
c1a1a
c1a1b
c1a1c
c1a1d
c1a1f
c1a2
c1a2a
c1a2b
c1a2c
c1a2d
c1a3
c1a3a
c1a3b
c1a3c
c1a3d
c1a4
c1a4b
c1a4d
c1a4e
c1a4f
c1a4g
c1a4i
c1a4j
c1a4k
c2a1
c2a1a
c2a1b
c2a2
c2a2a
c2a2b1
c2a2b2
c2a2c
c2a2d
c2a2e
c2a2f
c2a2g
c2a2h
c2a3
c2a3a
c2a3b
c2a3c
c2a3d
c2a3e
c2a4
c2a4a
c2a4b
c2a4c
c2a4d
c2a4e
c2a4f
c2a4g
c2a5
c2a5a
c2a5b
c2a5c
c2a5d
c2a5e
c2a5f
c2a5g
c2a5w
c2a5x
c3a1
c3a1a
c3a1b
c3a2
c3a2a
c3a2b
c3a2c
c3a2d
c3a2e
c3a2f
c4a1
c4a1a
c4a1b
c4a1c
c4a1d
c4a1e
c4a1f
c4a2
c4a2a
c4a2b
c4a3
c5a1

Multiplayer/Deathmatch
boot_camp
bounce
datacore
lambda_bunker
snark_pit
stalkyard
subtransit
undertow

Hazard course
t0a0
t0a0a
t0a0b
t0a0b1
t0a0b2
t0a0c
t0a0d

Item name list:

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
weapon_quantumdestabilizer

Cheat mode (alternate):
Start the game with one of the following command line parameters to enable that particular cheat code.

Monster creation:
You must be within the loading area of the desired monster for this cheat to work. For example, you cannot load up Barney while fighting the Nihilanth. Enter give <monster name> and the corresponding monster will appear. REMEMBER: Activate the /noclip first, or you will be stuck inside the monster.

People
monster_human_grunt
monster_barney
monster_scientist
monster_generic
monster_gman

Monsters
monster_sentery
monster_headcrab
monster_alien_slave
monster_alien_grunt
monster_bullchicken
monster_zombie
monster_apache
monster_ichthyosaur
monster_bigmomma
monster_barnacle
monster_tentacle
monster_gargantuan
monster_nihilanth
monster_houndeye
xen_tree
xen_spore_small
xen_spore_medium
xen_spore_large

Corpses
monster_barney_dead
monster_scientist_dead
monster_hevsuit_dead

Key binding:
If you bind the codes to keys you will be able to enable them easier. codes easier.  Try typing  /bind v "impulse 101" at the console window to bind the "HEV suit, weapons, and ammo " code to the "V" key. You can repeat this measure for other codes

HEV suit falls out:
Activate the /impulse 101 code while in mid-air and an error saying the hev suit has fallen out will appear.
 

Saving ammunition:
Don't waste precious pistol ammo on headcrabs. Use your crowbar, as it is more useful.
 

Using the pistol:
Use the following tricks to save ammo for all guns other than pistol. Most creatures or persons can be killed with five or less direct shots of the pistol.
 

Crabs: 2 shots or 1 crowbar hit
Zombies: 4 shots
Electric Aliens: 3 shots
Trip Bomb: 1 shot
Machinegun on stand: 5 shots
Soldiers: 5 shots
Dogs: 3 shots
Spitters: 4 shots