(source)

Enable cheats

When you enabled and brought up the console enter:

sv_cheats 1

Where the number "1" intends that cheats will be enabled.

to deactivate cheats just write a "0" instead of a "1".

Be aware, not all of the commands mentioned in this guide require
you to enable cheats [[b]sv_cheats 1[/b]], try without activating cheats at first.
The "Give" command
Just write "give" and enter the rest of the command.

For example:

give weapon_ar2

Item spawn commands

give item_healthkit Gives you a health kit
give item_healthvial Gives you a health vial
give item_suit Gives you the Hazardous enviroment suit
give item_battery Gives you a battery
give item_healthcharger
give item_suitcharger
give item_box_srounds 9mm Ammo
give item_box_mrounds SMG Ammo
give item_box_lrounds Pulse Gun Ammo
give item_box_buckshot Shotgun Ammo
give item_ar2_grenade SMG Grenades (secondary shot)
give item_ammo_crossbow Crossbow Ammo
give item_ammo_pistol Gives you pistol ammo
give item_ammo_357 Gives you 357. Magnum ammo
give item_ammo_smg1 Gives you SMG ammo
give item_ammo_smg1_grenade Gives you grenade launcher ammo for the SMG
give item_ammo_ar2 Gives you Overwatch Pulse Rifle ammo
give item_ammo_ar2_altfire Spawns energy grenade for the Overwatch Pulse Rifle
give item_rpg_round OR give item_ml_grenade Spawns grenades for the RPG
ent_create item_box_flare_rounds Spawns flare rounds at where you aim
ent_create item_box_sniper_rounds Spawns sniper rounds at where you aim
givecurrentammo Gives you ammo for the current weapon

If you want to remove an item, simply do this command

ent_remove

followed by (a space and) the command for that item (entity).

The flare ammo can't be picked up, but the sniper ammo can. Does this mean it is possible to use the sniper as a weapon after all? The answer is no, however both the flaregun and the sniper has been weapons in the beta version of HL2. What a mystery! Why was these not removed? Only valve knows why.

Weapon cheats

Enable cheats and use the “give” command.

weapon_crowbar
weapon_physcannon
weapon_stunstick (Adds 7% suit power instead)
weapon_portalgun (Portal only)
weapon_pistol
weapon_357
weapon_alyxgun (Pick up the gun to use it)
weapon_smg1
weapon_ar2
weapon_shotgun
weapon_crossbow
weapon_frag
weapon_rpg
weapon_hopwire (Worked in HL2:EP1 before but now only in Portal)
weapon_striderbuster (Works in EP1 or EP2)
weapon_bugbait
weapon_annabelle (A rifle that uses magnum ammo)
weapon_citizensuitcase
weapon_citizenpackage
weapon_oldmanharpoon (Works only in HL2: Lost Coast)
weapon_cubemap Gives you a tool mostly used by level designers

Gives you all the non-secret HL2 weapons- Don't type the give command in front of it.

impulse 101

If you want to remove a certain weapon:

ent_remove weapon_crossbow

This was an example of removing a crossbow, that you didn't want to keep.

Change max Ammo & Weapon Damage

Enable cheats and then input:

sk_max_357 # (The numerical value for # is new max ammo for .357 Magnum)

sk_max_ar2 # (The numerical value for # is new max ammo for Pulse Rifle)

sk_max_ar2_altfire # (The numerical value for # is new max ammo for Pulse Rifle energy orbs)

sk_max_buckshot # (The numerical value for # is new max ammo for shotgun)

sk_max_crossbow # (The numerical value for # is new max ammo for crossbow)

sk_max_grenade # (The numerical value for # is new max ammo for hand grenades)

sk_max_pistol # (The numerical value for # is new max ammo for pistol)

sk_max_rpg_round # (The numerical value for # is new max ammo for RPG)

sk_max_smg1 # (The numerical value for # is new max ammo for submachine gun)

sk_max_smg1_grenade # (The numerical value for # is new max ammo for SMG grenades)

sk_plr_dmg_pistol # (The numerical value for # is new damage value for pistol)

sk_plr_dmg_357 # (The numerical value for # is new damage value for .357 Magnum)

sk_plr_dmg_ar2 # (The numerical value for # is new damage value for Pulse Rifle)

sk_plr_dmg_buckshot # (The numerical value for # is new damage value for shotgun)

sk_plr_dmg_crossbow # (The numerical value for # is new damage value for crossbow)

sk_plr_dmg_crowbar # (The numerical value for # is new damage value for crowbar)

sk_plr_dmg_grenade # (The numerical value for # is new damage value for grenades)

sk_plr_dmg_rpg_round # The numerical value for # is new damage value for RPG rockets)

sk_plr_dmg_smg1 # (The numerical value for # is new damage value for submachine gun)

sk_plr_dmg_smg1_grenade # (The numerical value for # is new damage value for SMG grenades)

Observe: Giving weapon damage too much damage can freeze the game.

H.U.D Head Up Display commands

hidehud 5 (All hud will be gone)
hidehud 4 (All hud will be gone)
hidehud 3 (Health, suit power, and stamina will only show)
hidehud 2 (Health, suit power, and stamina will only show)
hidehud 1 (Health, suit power, and stamina will only show)
hidehud 0 (all hud will show - normal)

an alternative way to hide the hud completely are by typing:

cl_drawhud 0

putting 1 instead of 0 (zero) will make the hud visible again.

NPC Spawn commands

Type npc_create npc_ [name] to spawn an NPC. Here is a list of the names you can enter for [name]:

alyx (Alyx Vance)
barney (Barney Calhoun)
dog (Dog)
kleiner (Dr. Kleiner)
eli (Eli Vance)
mossman (Judith Mossman)
monk (Father Grigori)
citizen (Citizen)
headcrab (Regular Headcrab)
zombie (Regular Zombie)
zombie_torso (Regular Zombie Torso)
headcrab_fast (Fast Headcrab)
headcrab_black (Black Headcrab)
headcrab_poison (Poison Headcrab)
fastzombie (Fast Zombie)
poisonzombie (Poison Zombie)
zombine (Zombine that uses an error model, so it works better in HL2:EP1)
antlion (Antlion)
antlionguard (Antlion Guard)
combine_s (Combine Soldier/Combine Elite/Prison Guard)
metropolice (Metro Cop)
stalker (Stalker)
cscanner (City Scanner)
clawscanner (Claw Scanner)
manhack (Manhack)
rollermine (Rollermine)
combine_mine (Hopper Mine)
combine_camera" (Combine Camera)
turret_floor (Combine Turret)
turret_ceiling (Ceiling Turret)
turret_ground (Ground Turret)
launcher (?)
combinedropship (Dropship)
combinegunship (Gunship)
strider (Strider)
pigeon (Pigeon)
crow (Crow)
seagull (Seagull)
barnacle (Barnacle)
ichthyosaur (Ichthyosaur)
vortigaunt (Vortigaunt)
sniper (Sniper)

Aim at it and type this:

ent_rotate 90

This makes the npc sniper rotate to an another direction. This also works with other npcs as well.

npc_create_equipment weapon_stunstick

This command will allow the next npc you spawn, to be able to use a weapon (in this case a stunstick). This does not work on all npcs, just some of them. You can give them other different weapons. If you want to know the other commands to the other weapons, then you have to scroll to the weapon command list to see.

There is a problem when spawning hopper mine and pigeon (as far as I've tested)
Other NPC commands

npc_create_equipment weapon_shotgun (NPC spawns equipped with a shotgun.)
npc_create_equipment weapon_smg1 (NPC spawns equipped with a SMG.)
npc_create_equipment weapon_ar2 (NPC spawns equipped with a pulse rifle.)
npc_create_equipment weapon_crowbar (NPC spawns equipped with a crowbar.)
npc_create_equipment weapon_RPG (NPC spawns equipped with a RPG rocket launcher.)

violence_hblood 0 (Turns off human blood.)
violence_hgibs 0 (Turns off human gibs.)
violence_ablood 0 (Turns off alien blood.)
violence_agibs 0 (Turns off alien gibs.)

Gravity cheats

sv_gravity #number (600 = default)

The higher the number is, the more heavier the gravity gets!
The fewer the number is, the more lighter the gravity gets!

WARNING:
Taking the number too high, could possibly crash the game, so be aware of that!
Chapters - Unlock chapters
sv_unlockedchapters #

instead of "#" put the number of the chapters you want to unlock

# = somewhere between: 1-15 (whole numbers).

(Note: 15 is end credits)

Maps - Start a specific map

Write "map" followed by the name of the specific map you want to load:

intro Starts you at the games Intro.
Point Insertion

d1_trainstation_01 Spawns you in train station
d1_trainstation_02 Spawns you in train station after the first load point.
d1_trainstation_03 Spawns in the train station after the second load point.
"A Red Letter Day"
d1_trainstation_05 Spawns in the train station after the third load point.
d1_trainstation_06 Spawns you near the end of the train station.
Route Kanal
d1_canals_01 Spawns you in the canals.
d1_canals_01a Spawns you deeper in the canals.
d1_canals_02 Spawns in you in the canals after the second load point.
d1_canals_03 Spawns in you in the canals after the third load point.
d1_canals_04 Spawns in you in the canals after the fourth load point.
d1_canals_05 Spawns in you in the canals after the fifth load point.
Water Hazard
d1_canals_06 Spawns in you in the canals after the fifth load point.
d1_canals_07 Spawns in you in the canals after the sixth load point.
d1_canal_08 Spawns in you in the canals after the seventh load point.
d1_canal_09 Spawns in you in the canals after the eighth load point.
d1_canal_10 Spawns in you in the canals after the ninth load point.
d1_canals_11 Spawns in you in the canals after the tenth load point.
d1_canals_12 Spawns in you in the canals after the eleventh load point.
d1_canals_13 Spawns in you in the canals close to the end.
Black Mesa East
d1_eli_02 Spawns you in Eli's Lab.
d1_eli_01 Spawns you after the canal, outside of Eli's Lab.
"We don't go to Ravenholm"
d1_town_01 Spawns you in Ravenholm
d1_town_01a Spawns you in Ravenholm after the first load point area.
d1_town_02 Spawns you in Ravenholm after the secondload point area and later just before the graveyard fight.
d1_town_03 Spawns you in Ravenholm after the third load point area.
d1_town_02a Spawns you in Ravenholm just before the graveyard zombie fight.
d1_town_04 Spawns you in Ravenholm in the mines.
d1_town_05 Spawns you near the end of the Mines.
Highway 17
d2_coast_01 Spawns you on the coast (Vehicle)
d2_coast_02 Spawns you in the coast area after the first load point.
d2_coast_03 Spawns you in the coast area after the second load point.
d2_coast_04 Spawns you in the coast area after the third load point.
d2_coast_05 Spawns you in the coast area after the fourth load point.
d2_coast_06 Spawns you in the coast area after the fifth load point.
d2_coast_07 Spawns you in the coast area when you come, and go over THE bridge.
d2_coast_08 Spawns you in the coast area at the THE bridge.
Sandtraps
d2_coast_09 Spawns you in the coast area after the eighth load point.
d2_coast_10 Spawns you in the coast area after the ninth load point.
d2_coast_11 Spawns you in the coast area after the tenth load point.
d2_coast_12 Spawns you near the end of the coast area.
d2_prison_01 Spawns you outside Nova Prospekt
Nova Prospekt
d2_prison_02 Spawns you in Nova Prospekt after the first load point.
d2_prison_03 Spawns you in Nova Prospekt after the second load point.
d2_prison_04 Spawns you in Nova Prospekt after the third load point.
d2_prison_05 Spawns you in Nova Prospekt near the end of the level.
Entanglement
d2_prison_06 Spawns you in Nova Prospekt after the fifth load point.
d2_prison_07 Spawns you in Nova Prospekt after the sixth load point.
d2_prison_08 Spawns you near the end of Nova Prospekt.
d3_c17_01 Spawns you in city 17: In the lab one week later.
Anticitizen One
d3_c17_02 Spawns you in City 17 after the first load point.
d3_c17_03 Spawns you in City 17 after the second load point.
d3_c17_04 Spawns you in City 17 after the third load point.
d3_c17_05 Spawns you in City 17 after the fourth load point.
d3_c17_06a Spawns you in City 17 after the fifth load point. (In the manhacks tunnels)
d3_c17_06b Spawns you in City 17 after the sixth load point. (In the manhacks tunnels)
d3_c17_07 Spawns you in City 17 after the seventh load point.
d3_c17_08 Spawns you in City 17 after the eighth load point.
"Follow Freeman!"
d3_c17_09 Spawns you in City 17 after ninth the load point.
d3_c17_10a Spawns you in City 17 after the tenth load point.
d3_c17_10b Spawns you in City 17 after the eleventh load point.
d3_c17_11 Spawns you in City 17 after the twelfth load point.
d3_c17_12 Spawns you in City 17 after the thirteenth load point.
d3_c17_12b Spawns you in City 17 after the fourteenth load point.
d3_c17_13 Spawns you near the end of City 17 and beginning of the Citadel.
Our Benefactors
d3_citadel_01 Spawns you near the beginning of the Citadel.
d3_citadel_02 Spawns you after the first load point in the Citadel.
d3_citadel_03 Spawns you after the second load point in the Citadel.
d3_citadel_04 Spawns you after the third load point in the Citadel.
d3_citadel_05 Spawns you at the end of the chapter.
Dark Energy
d3_breen_01 Spawns you in the last map.
credits Starts you at the credits

Here is a list with bonus maps:
background01 Background of the square outside the trainstation.
background02 Background of the water canals.
background03 Background of Ravenholm.
background04 Background of Highway 17 / The Bridge.
background05 Background of Nova Prospekt.
background06 Background of the destroyed City 17.
background07 Background of the inside of the Citadel.
d3_c17_02_camera Same as d3_c17_02 just without weapons.
sdk_shader_samples Probably a demo map demostrating shaders.

Timescale cheats
This commands makes you able to change speed of how fast the game should be.

host_timescale 3.0 (Triple the faster speed)
host_timescale 2.0 (Double the faster speed)
host_timescale 1.0 (Normal speed)
host_timescale 0.5 (half speed)
host_timescale 0.25 (1/4 speed)
host_timescale 0.1 (Slow-mo speed)

You can make the game faster to add higher number, but also slower if you add lower numbers!

OBSERVE:
Taking the numbers too high can freeze the game in an instant, so avoid adding high numbers.

Note:
The faster the game goes, the fewer fps you get ingame.

Mixed cheats & commands

No damage: god
You never die, but you can get damaged: buddha
Infinite power for your suit: sv_infinite_aux_power 1
All weapons and ammo: impulse 101
Head skulls: impulse 102
Suicide: kill
Kill all npc’s in the area: npc_kill
Npcs don’t notice you: notarget (may make some scripted events not working correctly)
Become non-solid and fly like a ghost: noclip
Spawn a jeep: ch_createjeep
Spawn a jeep: impulse 82
Spawn an airboat: ch_createairboat
Add and enable the gun on the airboat: ent_fire prop_vehicle_airboat enablegun 1
Spawn an airboat: impulse 83
Super charged Gravity gun: physcannon_mega_enabled 1
Freeze npcs movements/animations: ai_disable (may make some scripted events not working correctly)
Make the camera shake a bit: shake
Delete an item that you are aiming at: impulse 203
Delete more than just an item that you are aiming at: ent_remove
Ignite almost anything you look at: ent_fire !picker ignite
Remove a special type of npc: ent_remove npc_*insert npc name here*
Remove all special type of npcs: ent_remove_all npc_*insert npc name here*
Adjust the noclip speed: sv_noclipspeed # (#=Number)
Adjust the physcannon push(launching) force or drag force: physcannon_maxforce # (#=Number)