Left 4 Dead Cheats

Developers Console 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:

give ammo: Full Ammunition
give pistol: Get Pistol
z_spawn weapon_SMG: Gives you the SMG
god: Invincibility
noclip: Toggles No Clipping Mode
sv_infinite_ammo 1: Unlimited Ammunition

Steam Achievements
Complete each requirement to receive the achievements.

A RIDE DENIED: Kill a Jockey within 2 seconds of it jumping on a Survivor.
A SPITTLE HELP FROM MY FRIENDS: As the Spitter, spit on a Survivor being choked by a Smoker.
ACID REFLEX: Kill a Spitter before she is able to spit.
ARMORY OF ONE: Deploy an ammo upgrade and have your team use it.
BACK IN THE SADDLE: As the Jockey, ride the Survivors twice in a single life.
BEAT THE RUSH: In a Survival match, get a medal only using melee weapons.
BRIDGE BURNER: Survive the Parish campaign.
BRIDGE OVER TREBLED SLAUGHTER: Cross the bridge finale in less than 3 minutes.
BURNING SENSATION: Ignite 50 Common Infected with incendiary ammo.
CACHE AND CARRY: Collect 15 gas cans in a single Scavenge match.
CHAIN OF COMMAND: Kill 100 Common Infected with the chainsaw.
CL0WND: Honk the noses of 10 Clowns.
CLUB DEAD: Use every melee weapon to kill Common Infected.
CONFEDERACY OF CRUNCHES: Finish a campaign using only melee weapons.
CRASS MENAGERIE: Kill one of each Uncommon Infected.
DEAD IN THE WATER: Kill 10 swampy Mudmen while they are in the water.
DISMEMBERMENT PLAN: Kill 15 Infected with a single grenade launcher blast.
FRIED PIPER: Use a Molotov to burn a Clown leading at least 10 Common Infected.
FUEL CRISIS: Make a Survivor drop a gas can during overtime.
GAS GUZZLER: Collect 100 gas cans in Scavenge.
GAS SHORTAGE: Cause 25 gas can drops as a Special Infected.
GONG SHOW: Prove you are stronger than Moustachio.
GUARDIN' GNOME: Rescue Gnome Chompski from the Carnival.
HEAD HONCHO: Decapitate 200 Infected with a melee weapon.
HEARTWARMER: In a Versus match, leave the saferoom to defibrillate a dead teammate.
HUNTING PARTY: Win a game of Scavenge.
LEVEL A CHARGE: Kill a Charger with a melee weapon while they are charging someone.
LONG DISTANCE CARRIER: As the Charger, grab a Survivor and carry them over 80 feet.
MEAT TENDERIZER: As the Charger, grab a Survivor and smash them into the ground for a solid 15 seconds.
MIDNIGHT RIDER: Survive the Dark Carnival campaign.
PRICE CHOPPER: Survive the Dead Center campaign.
QUALIFIED RIDE: As the Jockey, ride a Survivor for more than 8 seconds.
RAGIN' CAJUN: Survive the Swamp Fever campaign.
ROBBED ZOMBIE: Collect 10 vials of Boomer vomit from infected CEDA agents you have killed.
RODE HARD, PUT AWAY WET: As the Jockey, ride a Survivor and steer them into a Spitter's acid patch.
SCATTERING RAM: As the Charger, bowl through the entire enemy team in a single charge.
SCAVENGER HUNT: Stop the enemy team from collecting any gas cans during a Scavenge match.
SEPTIC TANK: Use a bile grenade on a Tank.
SHOCK JOCK: Revive 10 dead Survivors with the defibrillator.
SOB STORY: Navigate the sugar mill and reach the safe room without killing any Witches.
SPIT HAPPENS: As the Spitter, hit every Survivor with a single acid patch.
STACHE WHACKER: Prove you are faster than Moustachio.
STILL SOMETHING TO PROVE: Survive all campaigns on Expert.
STRENGTH IN NUMBERS: Form a team and beat an enemy team in 4v4 Versus or Scavenge.
TANK BURGER: Kill a Tank with melee weapons.
THE QUICK AND THE DEAD: Revive 10 incapacitated Survivors while under the speed boosting effects of adrenaline.
THE REAL DEAL: Survive a campaign on Expert difficulty with Realism mode enabled.
VIOLENCE IN SILENCE: Navigate the impound lot and reach the cemetary safe room without tripping any alarms.
WEATHERMAN: Survive the Hard Rain campaign.
WING AND A PRAYER: Defend yourself at the crashed airliner without taking damage.