At the main menu, select "Options," then "Keyboard/Mouse." Enable "Allow Developers Console." During gameplay, press "~" to access the console. Type in "Sv_Cheats 1" and press "Enter" to enable the cheats. Then, enter any one of the following:
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god -- Invincibility

sv_infinite_ammo 1 -- Unlimited Ammunition

noclip -- Toggles No Clipping Mode

give pistol -- Get Pistol

give ammo -- Full Ammunition

z_spawn weapon_SMG -- Gives you the SMG.

z_spawn mob -- spawns a zombie horde

z_spawn -- spawns a special infected (infected names:hunter,boomer,smoker,witch,tank)

give health -- gives 100 health

give hunting_rifle -- gives hunting rifle

give molotov -- gives molotov cocktail

give autoshotgun -- Gives auto shotgun

give health -- gives full health

z_spawn tank -- Spawns tank at location of crosshair

z_spawn boomer -- Spawns boomer at location of crosshair

z_spawn smoker -- Spawns smoker at location of crosshair

z_spawn hunter -- Spawns hunter at location of crosshair

give pain_pills -- Gives pain pills

thirdperson -- Sets the camera to be in third person mode.

thirdperson_mayamode -- Sets the camera to be fixed in place while in either third person or third person shoulder mode. Enter once to turn on and enter again to turn off.

thirdpersonshoulder -- Sets the camera behind the shoulder in third person.

give rifle -- Gives you an Mk 47 (machine gun)

z_health -- Change Zombie Health

z_speed -- Change Zombie Speed

director_panic_forever 0/1 -- Enables/Disables never ending panics.

z_common_limit -- Sets the maximum amount of regular zombies

director_force_panic -- Forces a panic event

z_spawn zombie -- Spawns 1 Zombie

give first_aid_kit -- gives you first aid kit

give pipe_bomb -- gives pipebomb

give molotov -- gives molotov

give gascan -- gives gascan

give oxygentank -- gives oxygen tank

give propanetank -- gives propanetank

fire -- Spawns a lighted molotov right on the floor under you.

boom -- Spawns a active pipebomb under you.

firstperson -- Play in first person (default); this cheat is only necessary if thirdperson mode is enabled

z_frustration_lifetime -- Sets the number of seconds until the player loses control of the Tank from not attacking survivors

z_tank_health -- Sets the maximum health of the Tank

z_witch_damage -- ...