Enter in-game in the console. To ativate console, go to options, Keyboard, Advanced, Enable Developer Console.

Code	Effect
sv_cheats 1	Activates cheats
notarget	Enemies do not target you
noclip	Fly through walls
give weapon_rpg	Give RPG
give weapon_357	Gives .357 magnum
impulse 101	Gives all weapons and full ammo
give weapon_alyxgun	Gives Alyxs Gun
give weapon_crossbow	Gives Crossbow
give weapon_crowbar	Gives crowbar
give weapon_physcannon	Gives Gravity Gun
give weapon_frag	Gives grenades
give weapon_annabelle	Gives Grigoris gun
npc_create_equipment weapon_357	Gives npc a .357 magnum when spawned (valid for Civil Protection and Citizens)
npc_create_equipment weapon_pistol	Gives npc a pistol when spawned (valid for Civil Protection and Citizens)
give weapon_bugbait	Gives Pheropods (will not make antlions on your side)
give weapon_pistol	Gives pistol
give weapon_ar2	Gives Pulse Rifle
give weapon_shotgun	Gives shotgun
give weapon_smg1	Gives SMG
god	Invincible
npc_create npc_alyx	Spawns Alyx
npc_create npc_antlion	Spawns Antlion
npc_create npc_antlionguard	Spawns Antlion Guard
npc_create npc_barney	Spawns Barney
npc_create npc_cscanner	Spawns City Scanner
npc_create npc_metropolice	Spawns Civil Protection
npc_create npc_crow	Spawns Crow
npc_create npc_combinedropship	Spawns Dropship
npc_create npc_eli	Spawns Eli
npc_create npc_headcrab_fast	Spawns Fast Headcrab
npc_create npc_fastzombie	Spawns Fast Zombie
npc_create npc_combinegunship	Spawns Gunship
npc_create npc_headcrab	Spawns Headcrab
npc_create npc_zombie	Spawns Headcrab Zombie
npc_create npc_zombie_torso	Spawns Headcrab Zombie Torso
npc_create npc_helicopter	Spawns Hunter-Chopper
npc_create npc_Kleiner	Spawns Isaac Kleiner
npc_create npc_sniper	Spawns Overwatch Sniper
npc_create npc_combine_s	Spawns Overwatch Soldier
npc_create npc_pigeon	Spawns Pigeon
npc_create npc_headcrab_black	Spawns Poison Headcrab
npc_create npc_headcrab_poison	Spawns Poison Headcrab
npc_create npc_poisonzombie	Spawns Poison Zombie
npc_create npc_citizen	Spawns Resistace member/citizen/Refugee depending on what chapter
npc_create npc_seagull	Spawns Seagull
npc_create npc_clawscanner	Spawns Shield Scanner
npc_create npc_strider	Spawns Strider (use noclip to get up to striders height)
npc_create_equipment weapon_rpg	Valid for Citizens
npc_create_equipment weapon_smg1	Valid for Civil Protection, Overwatch Soliders and Citizens
npc_create_equipment weapon_shotgun	Valid for Overwatch Soldiers, Citizens and Alyx
npc_create_equipment weapon_ar2	Valid for Overwatch Soldiers, Citizens, Alyx and Barney