Enter the "Options" menu. Select "Keyboard", then "Advanced", then turn on the "Enable Developer Console" option. Press ~ to display the console window, then enter sv_cheats 1and press ENTER to enable cheat mode. You can then activate the following cheats by entering the corresponding codes:

CHEAT FUNCTION - CHEAT CODE
Spawn .357 Magnum - give weapon_357
Spawn Alyx's Gun - give weapon_alyxgun
Spawn Grigori's gun - give weapon_annabelle
Spawn Pulse rifle - give weapon_ar2
Spawn Pheropods - give weapon_bugbait
Spawn crossbow - give weapon_crossbow
Spawn crowbar - give weapon_crowbar
Spawn grenades - give weapon_frag
Spawn Gravity Gun - give weapon_physcannon
Spawn pistol - give weapon_pistol
Spawn RPG - give weapon_rpg
Spawn shotgun - give weapon_shotgun
Spawn SMG - give weapon_smg1
Invincibility - god
Spawn all weapons and full ammo - impulse 101
Move through walls - noclip
Ignored by enemies - notarget
Spawn Alyx - npc_create npc_alyx
Spawn Antlion - npc_create npc_antlion
Spawn Antlion Guard - npc_create npc_antlionguard
Spawn Barney - npc_create npc_barney
Spawn Resistance member, citizen, or refugee depending on current chapter - npc_create npc_citizen
Spawn Shield Scanner - npc_create npc_clawscanner
Spawn Overwatch Soldier - npc_create npc_combine_s
Spawn Dropship - npc_create npc_combinedropship
Spawn Gunship - npc_create npc_combinegunship
Spawn Crow - npc_create npc_crow
Spawn City Scanner - npc_create npc_cscanner
Spawn Eli - npc_create npc_eli
Spawns Fast Zombie - npc_create npc_fastzombie
Spawn Headcrab - npc_create npc_headcrab
Spawn Poison Headcrab - npc_create npc_headcrab_black
Spawn Fast Headcrab - npc_create npc_headcrab_fast
Spawn Poison Headcrab - npc_create npc_headcrab_poison
Spawn Hunter-Chopper - npc_create npc_helicopter
Spawn Isaac Kleiner - npc_create npc_Kleiner
Spawn Civil Protection - npc_create npc_metropolice
Spawn pigeon - npc_create npc_pigeon
Spawn Poison Zombie - npc_create npc_poisonzombie
Spawn seagull - npc_create npc_seagull
Spawn Overwatch Sniper - npc_create npc_sniper
Spawn Strider - npc_create npc_strider
Spawn Headcrab Zombie - npc_create npc_zombie
Spawn Headcrab Zombie Torso - npc_create npc_zombie_torso
Give Civil Protection and Citizens NPC a .357 Magnum when spawned - npc_create_equipment weapon_357
Give Overwatch Soldiers NPC, Alyx, and Barney a AR2 when spawned - npc_create_equipment weapon_ar2
Give Civil Protection and Citizens NPC a pistol when spawned - npc_create_equipment weapon_pistol
Give Citizens NPC a RPG when spawned - npc_create_equipment weapon_rpg
Give Citizens, Overwatch Soldiers NPC, and Alyxa shotgun when spawned - npc_create_equipment weapon_shotgun
Give Civil Protection, Citizens, Overwatch Soldiers NPC a SMG Magnum when spawned - npc_create_equipment weapon_smg1
Enable use of wired Xbox 360 controller - exec 360controlle