Left 4 Dead Cheats

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 <#> -- ... 

#1: In Dead Air, before the finale, while still in the safe room, sometimes you can see a box which advertises Valve Software's "Team Fortress 2".(Mostly works on Xbox version)

#2:The APC in Blood Harvest that says "TK-421" is a reference to a stormtroopers in Star Wars that Luke Skywalker and Han solo incapacitated.

#3: At the Boathouse in Deathtoll, even on expert, a healthpack and pills spawn in the bathroom across the field from the house.

#4: You can see th burning city of Newburg from Dead Air at the end of Death Toll

#5: Tanks float when dead.

#6: The open area at the end of Death Toll is a great area to get the "Man Vs. Tank" Achievement. There is enough room to fend off the hoard yourself and enough to deal with the tank.

#7: John Slater, who voices the person who drives the boat in Death toll also voices the Sniper in Team Fortress 2.

#8: In the hospital levels of No Mercy, there are alot of X-rays with only 4 fingers, referencing to the cover art.

#9: On the movie poster for No mercy, Zoey has 3 hands holding her gun.(You have to look very closely.)

#10: If you are a Infected when the elevator in No mercy is activated and heading down, the number of what floor it is on will not change from "04" in reference to the title.

#11: In the hospital chapters starting safe room, there is graffiti on the wall saying "I killed..." and then listing different variables, including 53,594, the number of infected you have to kill to receive the "Zombie Genocide" Achievement.

#12: At the beginning of the sewers in No Mercy, if any gas tank catches fire, or shot, the entire station explodes into flames.

#13: Near a escalator in The subway, there is a ad for "Orange Box" Juice, referencing to one of Valves vastly popular game packages.

#14: In some of the newspapers that you can see throughout the game, Dr.Breen can be seen from "Half Life".

enable the console via the Options menu. Bring up the console and type "sv_cheats 1" to activate cheat mode. You may then type any of the following cheat codes.

Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

Helping Hand (20 points): Revive 50 incapacitated Survivors.
Field Medic (20 points): Heal 25 Survivors with a first aid kit.
Pharm-assist (20 points): Give pain pills to 10 Survivors.
My Bodyguard (15 points): Protect any Survivor from an attacking Infected 50 times.
Dead Stop (10 points): Punch a Hunter as he is pouncing.
Cr0wnd (30 points): Kill a Witch with a single headshot.
Untouchables (30 points): No Survivors take damage after contacting the rescue vehicle.
Drag and Drop (20 points): Rescue a Survivor from a Smoker's tongue before he takes damage.
Blind Luck (20 points): You or another Survivor take no damage after being vomited on by a Boomer.
Akimbo Assassin (30 points): Survive an entire campaign using only pistols.
Hero Closet (10 points): Rescue a Survivor trapped in a closet.
Hunter Punter (10 points): Shove a Hunter off of a pinned and helpless Survivor.
Tongue Twister (20 points): Free yourself from a Smoker who has grabbed you with his tongue.
No Smoking Section (15 points): Kill 10 Smokers as they are pulling helpless Survivors.
101 Cremations (20 points): Set 101 Infected on fire.
Do Not Disturb (20 points): Sneak past all Witches in a campaign without disturbing one.
Man vs Tank (30 points): Single-handedly kill a Tank.
Tankbusters (20 points): Kill a Tank without it dealing any damage to a Survivor.
Safety First (30 points): Play an entire campaign with no Survivors taking friendly fire damage.
No-one Left Behind (20 points): Beat a campaign with all 4 Survivors.
Unbreakable (30 points): Finish a campaign without ever being healed.
Witch Hunter (20 points): Kill a Witch without any Survivor taking damage from her.
Red Mist (20 points): Kill 1000 Infected with a mounted machine gun.
Pyrotechnician (20 points): Blow up 20 Infected in a single explosion.
Zombie Genocidest (20 points): Kill 53,595 Infected.
Dead Giveaway (10 points): Heal a fellow Survivor when your own health is below 10.
Stand Tall (20 points): Survive a campaign without being incapacitated.
Zombicidal Maniac (30 points): Survive any campaign on Expert.
What Are You Trying to Prove? (35 points): Survive all campaigns on Expert.
Nothing Special (30 points):... 

To insert the cheats below, call-up the in-game cheats console by pressing the tilde (~) key. Then, insert the cheats on the left for the corresponding result on the right.

Press ~ below Esc Button.
First type Sv_Cheats 1

Good For Single Player Play :D.