Activate the console by going to the keyboard options in the game and clicking on the Advanced controls.  Then, when you are in the game, press the ~ button to bring up the console.  Most of these cheats will require sv_cheats set to 1.

Half Life 2, Episodes 1 and 2, and Portal Commands

sv_cheats 1  - Activates cheats 

God  - God mode 

noclip  - fly 

notarget  - Enemies dont target you

Spawning NPCs

Access the console, type in "npc_create npc_<name>" and replace the name with one of the options below:

zombie - Spawns a Zombie.

zombine - Spawns a Zombine (Combine Zombie)

zombie_torso - Spawns a Zombie without legs. 

fastzombie - Spawns a Fast Zombie. 

crow - Spawns a Crow 

cscanner - Spawns a Combine Scanner. 

combine_s - Spawns a Combine Soldier. 

citizen - Spawns a City 17 Citizen. 

breen - Spawns Breen. 

barney - Spawns Barney. 

barnacle - Spawns a barnacle. 

antlion - Spawns an Antlion. 

alyx - Spawns Alyx. 

antlionguard - Spawns an Antlion Guard. 

stalker - Spawns a Stalker. 

strider - Spawns a Strider. 

vortigaunt - Spawns a Vortigaunt. 

turret_ceiling - Spawns a Ceiling Turret. 

seagull - Spawns a Seagull 

pigeon - Spawns a Pigeon. 

dog - Spawns Dog. 

mossman - Spawns Mossman 

monk - Spawns Father Gregori 

poisonzombie - Spawns a Poison Zombie. 

rollermine - Spawns a Rollermine. 

metropolice - Spawns a Combine Metrocop 

manhack - Spawns a Manhack 

launcher - Spawns a Headcrab Launcher 

kleiner - Spawns Kleiner 

ichthyosaur - Spawns a Ichthyosaur. 

headcrab_poison - Spawns a Poison Headcrab 

headcrab_black - Spawns a Poison Headcrab 

headcrab_fast - Spawns a Fast Headcrab 

headcrab - Spawns a Headcrab. 

helicopter - Spawns a Combine Helicopter 

gman - Spawns Gman 

eli - Spawns Eli 

combinegunship - Spawns Combine Gunship 

turret_floor - Spawns a Floor Turret. 

combinedropship - Spawns a Combine Dropship 

crabsynth - Spawns a Crab Synth 

mortarsynth - Spawns a Mortar Synth

For Half-Life 2, Episode One and Episode Two

give weapon_alyxgun - Gives you the ’Alyx Gun’ 

sk_max_alyxgun # - # is new max ammo for Alyx Gun (default 0) 

sk_plr_dmg_alyxgun # - # is new damage for Alyx Gun (default 150) 

sk_plr_dmg_357 # - # is new damage value for the .357 Magnum 

sk_plr_dmg_crossbow # - # is new damage value for the crossbow 

sk_plr_dmg_crowbar # - # is new damage value for the crowbar 

sk_plr_dmg_grenade # - # is new damage value for grenades 

sk_plr_dmg_pistol # - # is new damage value for the pistol 

sk_plr_dmg_ar2 # - # is new damage value for the Pulse Rifle 

sk_plr_dmg_rpg_round # - # is new damage value for the RPG 

sk_plr_dmg_buckshot # - # is new damage value for the shotgun 

sk_plr_dmg_smg1_grenade # - # is new damage value for the SMG’s grenades 

sk_plr_dmg_smg1 # - # is new damage value for the SMG 

sk_max_357 # - # is new max ammo for the .357 Magnum 

sk_max_crossbow # - # is new max ammo for the crossbow 

sk_max_grenade # - # is new max ammo for grenades 

sk_max_pistol # - # is new max ammo for the pistol 

sk_max_ar2 # - # is new max ammo for the Pulse Rifle 

sk_max_ar2_altfire # - # is new max ammo for the Pulse Rifle’s energy orbs 

sk_max_rpg_round # - # is new max ammo for the RPG 

sk_max_buckshot # - # is new max ammo for the Shotgun 

ai_disable - Turns Non-Player Characters off. Type again to resume.   

give item_healthkit - Gives you 25+ healthkit 

give item_battery - Gives you 15+ battery 

kill - Kills yourself (I hardly see use for this) 

sk_max_smg1_grenade # - # is new max ammo for SMG grenades 

sk_max_smg1 # - # is new max ammo for submachine gun 

weapon_357 - Gives you the .357 Magnum. 

weapon_shotgun - Gives you the shotgun 

weapon_pistol - Gives you the pistol 

net_graph 1 - a more thorough fps display 

viewmodel_fov # - Adjusts size of the weapon you’re carrying (54 = default) 

impulse 101 - Gives you all weapons 

mat_yuv 1 - Turn on Black and White mode 

mat_yuv 0 - Turn off Black and White mode 

cl_showpos 1 - Brings up position display in top right of screen (0 removes it) 

getpos - can be used to get current coords for use with setpos 

skill # - change skill level (# = 1, 2, or 3) 

air_density # - Change the density of air. (Add number in place of #) 

physcannon_maxforce # - Changes how hard you propel objects with the Gravity Gun 

physcannon_maxmass # - Changes how large pulled objects can be 

physcannon_pullforce # - Changes how quickly objects are pulled 

physcannon_tracelength # - Changes the length that objects can be pulled from 

physcannon_cone # - Changes the radius of the cone used to pick up objects 

host_timescale # - Changes the speed of the game (for slow-mo/fast-mo) (# < 1 = slower, # > 1 = faster) 

shake - Creates an earthquake. 

npc_create_aimed - Creates an NPC (Aiming away from player) 

weapon_crossbow - Gives you the crossbow 

weapon_crowbar - Gives you the crowbar 

Hurtme # - Damages player by whatever variable you input as # 

crosshair 0 - Disables Crosshair 

crosshair 1 - Enables Crosshair 

maps - Displays map listing 

mat_yuv 0 - Enables Color Mode 

mat_yuv 1 - Disables Color Mode 

+mlook - Enables mouse look 

cl_ragdoll_collide 1 - Enables ragdolls that don’t clip through each other, but stack realistically on one another. 

exec # - Execute a script file (Input filename in place of #) 

firstperson - First Person View 

weapon_frag - Gives you a frag grenade 

sv_infinite_aux_power 1 - Gives infinite power for sprinting, breating underwater, etc. 

sv_infinite_aux_power 0 - Takes away infinite power for sprinting, breating underwater, etc. 

r_screenoverlay effects/combine_binocoverlay.vmt - Gives you Combine vision during gameplay. 

give weapon_annabelle - Gives you Father Gregori’s modified shotgun 

give weapon_physcannon - Gives you the Gravity Gun 

give item_suit - Gives you the HEV suit 

item_healthkit - Gives you the large health kit (25HP) 

give item_healthvial - Gives you the small health vial (10HP) 

r_screenoverlay effects/tp_eyefx/tp_eyefx.vmt - Gives you weird vision during gameplay.  impulse 76 - Creates a human grunt. 

buddha - Health never goes below 1 

sk_plr_num_shotgun_pellets # - Increases pellets fired from each shotgun shell. 100 works well. 1000+ and it lags. 

give item_battery -   Increases suit charge by 15 points. 

mat_numtextureunits # - Limit the number of texture units. (Add number in place of #) (0=Default) 

ch_createjeep - Spawn a Jeep 

ch_createairboat - Spawn an airbot 

physcannon_megacannon 1 - Makes your Gravity Gun a Super Gravity Gun 

setpos - Move player to specified origin 

sk_npc_dmg_shotgun # - NPC’s do # damage with shotgun 

sk_npc_dmg_ar2 # - NPC’s do # damage with the pulse rifle 

sk_npc_dmg_smg1 # - NPC’s do # damage with the smg 

weapon_ar2 - Gives you the Overwatch Standard Issue Pulse Rifle 

weapon_bugbait - Spawns a Pherapods. 

mat_fullbright 1 - removes all shadows (a 0 restores them) 

impulse 203 - Removes the item or NPC you are pointed at 

help $ - replace $ with a command to get a brief description of it 

weapon_rpg - Gives you the RPG 

mat_depthbias_normal 1 - See through walls 

sv_gravity # - Set Gravity (Add number in place of #)dsp_explosion_effect_duration # 

dsp_explosion_effect_duration # - Set length of confusion/ear-ringing effect(Add number in place of #) 

give item_rpg_round - Spawns grenades for the RPG 

give item_ml_grenade - Spawns grenades for the RPG 

sv_stopspeed # - Set Minimum Stopping Speed on ground (Add number in place of #) 

sv_waterdist # - Set Vertical View when eyes are near water plane. 

sv_friction # - Set World Friction (Add number in place of #) 

sv_bounce # - Sets bounce multiplier for physically simulated object collisions 

Developer 0 - Sets developer mode to off 

Developer 1 - Sets developer mdoe to on 

Developer 2 - Sets developer mode to verbose 

sv_maxvelocity # - Sets Maximum Velocity of any moving object 

status - shows some game info, incl. current map 

setang - Snap players eyes to a specified pitch yaw 

impulse 82 - Spawns a Jeep 

impulse 83 - Spawns an Airboat 

give item_ammo_357 - Spawns ammo for the .357 magnum 

give item_box_srounds - Spawns ammo for the Pistol 

give item_ammo_crossbow - Spawns ammo for the Crossbow 

give item_ammo_ar2_altfire - Spawns energy grenades for the Overwatch Pulse Rifle 

stop - Stops recording the current demo. 

play demo_name - Plays demo.(change demo_name to the demo you want to be played) 

record demo_name - Records a demo of all the user’s actions within the game until the "stop" command is issued. 

sv_unlockedchapters # - Unlocks Chapters 1 through # 

cl_drawhud 0 - Turns off your HUD 

cl_enablehud 0 - Turns off your HUD 

sv_soundemitter_filecheck - Toggle reporting missing .wav files for sounds 

cl_enablehud 1 - Turns on your HUD 

cl_drawhud 1 - Turns on your HUD   

thirdperson - Third Person View 

weapon_smg1 - Gives you the SMG 

give item_box_buckshot - Spawns shells for the 12-gauge shotgun 

give item_box_mrounds - Spawns primary fire ammo for the SMG 

give item_ammo_ar2 - Spawns primary fire ammo for the Overwatch Pulse Rifle 

give item_ar2_grenade - Spawns grenades for the SMG (strangely enough) 

givecurrentammo - Gives full ammo for current weapon. Only refills secondary ammo if that was fired most recently. Cannot refill empty weapons.