Left 4 Dead Cheats

Drag and Drop (20): Rescue a Survivor from a Smoker's tongue before he takes damage.
Field Medic (20): Heal 25 Survivors with a first aid kit.
Grim Reaper (15): Survive the Blood Harvest campaign.
Ground Cover (30): Save another Survivor from a Special Infected while on the ground.
Helping Hand (20): Revive 50 incapacitated Survivors.
Hero Closet (10): Rescue a Survivor trapped in a closet.
Hunter Punter (10): Shove a Hunter off of a pinned and helpless Survivor.
Jump Shot (20): Headshot a Hunter while he's leaping.
Lamb 2 Slaughter (20): As an Infected, incap a Survivor who has entered and left a safe room.
Man vs Tank (30): Single-handedly kill a Tank.
Mercy Killer (15): Survive the No Mercy campaign.
My Bodyguard (15): Protect any Survivor from an attacking Infected 50 times.
No Smoking Section (15): Kill 10 Smokers as they are pulling helpless Survivors.
No-one Left Behind (20): Beat a campaign with all 4 Survivors.
Nothing Special (30): Survive a campaign with no Survivors taking damage from Special Infected.
Pharm-assist (20): Give pain pills to 10 Survivors.
Pyrotechnician (20): Blow up 20 Infected in a single explosion.
Red Mist (20): Kill 1000 Infected with a mounted machine gun.
Safety First (30): Play an entire campaign with no Survivors taking friendly fire damage.
Spinal Tap (10): Kill an Infected with a single blow from behind.
Stand Tall (20): Survive a campaign without being incapacitated.
Stomach Upset (20): All Survivors complete a campaign without being vomited on.
Tankbusters (20): Kill a Tank without it dealing any damage to a Survivor.
Toll Collector (15): Survive the Death Toll campaign.
Tongue Twister (20): Free yourself from a Smoker who has grabbed you with his tongue.
Towering Inferno (10): Light a Tank with a Molotov.
Unbreakable (30): Finish a campaign without ever being healed.
Untouchables (30): No Survivors take damage after contacting the rescue vehicle.
What Are You Trying to Prove? (35): Survive all campaigns on Expert.
Witch Hunter (20): Kill a Witch without any Survivor taking damage from her.
Zombicidal Maniac (30): Survive any campaign on Expert.
Zombie Genocidest (20): Kill 53,595 Infected.

Akimbo Assassin Achievement Made Easy
The Akimbo Assassin Achievement requires you to play an entire mission with only the pistol(s). One easy way to complete this is to start a mission, shoot a few zombies with your pistol only, then choose "Take a Break" and let the NPCs finish the level.

Fire. And Lots Of It.
The infected cannot extinguish fires once they are ignited (all modes - but especially in versus mode). Use that to your advantage in the campaign by using the molotov (or igniting gas cans) on tough targets like Tanks and Witches.

Speed Life
As the director's commentary points out, the Tank can be outrun by any survivor with green (51% or more) health, but the Tank can overtake any survivor with 50% of less life, because the more injured a survivor is, the slower s/he moves.

Pain pills will boost the speed of an injured survivor, although the color of the life bar will reflect his/her original health status.

Unbreakable / Stand Tall Achievements
Do these two achievements on Easy difficulty (duh!) and get into the habit of running back to let other players (or bots, if on single player) take out the Tanks and enemies. On single-player mode, if a bot accidentally heals you, you can die, restart the map, and not have that count against your achievement. However, if you are playing online, that exploit will not work.

Cr0wned Achievement
To kill a witch with a headshot, put the game on Easy difficulty, acquire an auto-shotgun, and look for a witch. The player going after this achievement should simply (and quickly) approach the witch (lights off), causing her to rise. As the witch is standing (she is slower to attack on easy), unload all 10 shells into her head.

Reloaded Defence
When reloading any weapon, your survivor can keep attacking with the melee button. The reloading sequence will continue unabated.

Hot Swap
If you need to 'reset' your primary weapon to its original (cherry) state, pick up a weapon different from yours, then re-select the weapon you want. This is faster than reloading (plus the time used from replenishing from the ammunition pile), and can help in close-call rescue finales.

Respawn for Max Health in Vs.
If you'd like to "refill" your health as a zombie, run away from the survivors until you see a message claiming you are too far away. Press X, and then you'll be allowed to respawn back in the fray. Hopefully they won't make it to the safe room before you're back!

Unbreakable Achievement
The Unbreakable Achievement requires you to not heal yourself or be healed throughout an entire mission. If the computer begins to heal you, whip out your own health pack (don't heal yourself though!) and it will stop the computer in its tracks.

Rocket Launcher
Complete the game on any difficulty. It will appear on any lvl at the begining and middle of each part.

Rocket Launcher: Complete the game on any difficulty.