xbox360-Left 4 Dead

Invincibility while reloading
Press B to reload, then hold LT to knock back attacking zombies while reloading your gun. -From: Ryan Helgeson

Fast melee
Rapidly tap LT, Y repeatedly. This will double your melee speed. This helps to complete the game on the Expert difficulty from hordes of attacking zombies.

Safe area
When hordes of Infected appear, try to find a room with a door and no windows. Go inside, and close the door. The Infected may break the door, but when you see a hole in it, shoot the Infected trying to break it down. Your teammates can handle most of the Infected.

Unbreakable door
If you are low on health and need somewhere to hide, find a bathroom stall. Go inside the stall, and close the door. The door never breaks down. This works well if you are trying to heal yourself.

Easy wins as Infected in Blood Harvest: The Tunnel
There is a warehouse-type area immediately after the starting safe room. Quickly run back to the emergency door on the other side of the warehouse. There should be a forklift near the door. Spawn and push this forklift in front of the doorway. Note: You may need to break the white door in front of the emergency door to get the forklift all the way in front of the doorway.

Shortcut in Dead Air: The Construction Site
There is a metal door and a raised fence by the barricade that you burn. Jump on the fence, and follow it until it begins to go downward. At this point, jump on the metal siding door. Note: Some practice may be required.

Shortcut in Dead Air: The Crane
Jump on the fence after climbing on the vent cover. Go across the fence, then jump to the other side of the horizontal fence. Turn left, and go to the end. Jump up and to the right to the air conditioning unit box on the corner of the next building. Once on the next building, go to the left corner. Jump to the top branch of the tree below you. You will lose a lot of health after landing; however, you will be at the next safe room.

Shortcut in No Mercy: The Apartments
As soon as the level starts, look over the edge of the building and down to the street. Notice the box-like object that is partway down. Drop down to it. Although you will take some damage, you will still be alive. You can then drop down to the ground and continue the level.

Easy wins as Infected in No Mercy: The Hospital
Before you spawn, go all the way up to floor 28, where the elevator lets out. Just outside of the elevator is a generator. Move it in front of the elevator door. If the Survivors know what you are doing, they can move it. It requires three survivors to move an object. However, if you have someone else on the Infected side, they can go a little bit further and move the forklift in front of the doorway.

Easy kills as Infected in No Mercy: The Sewer
The Survivors must jump into a sewer hole at a certain point. As an Infected, use this to your advantage. While playing as a Smoker or Hunter, hide behind one of the large canisters in the room. Once one of the Survivors open the hole and are done checking the vicinity, they will jump in. Due to the fact they must jump in one at a time or get stuck, make sure you have a clear view of the last one in line. Once all three are down and the fourth Survivor is still up, pounce or grab him or her. By doing this, he or she will not be able to be saved, unless someone throws a Molotov cocktail or pipe bomb.

Easy wins as Infected in No Mercy: The Sewer
In the warehouse near the end of the level, immediately after the area with the gas station and the lift, there is a forklift which you can use to block the door the Survivors must go through. When the level starts, do not spawn. Instead, quickly go to the warehouse. When you get there, spawn and start pushing the forklift to the door. To push the forklift, keep attacking it. It will move very slowly, but you will have time to push it before the Survivors reach it. The only way the Survivors can move it is by throwing pipe bombs at it repeatedly.

This trick works best as a Hunter. Before you spawn in the No Mercy campaign, Level 3, go all the way to the very end, at the hospital. Once up the ladder, turn to your right, and there should be a dumpster. Move it all the way over to the manhole, and one of two things will happen. It will either fall all the way down, which is bad because the Survivors cannot move it; or it will partially fall in. The survivors cannot move this at all. If you are a Tank, you can move a car over it, which is better than the dumpster. From here, do not puke on them as a Boomer, as it will attract the Horde. The Horde will try to move the object away from the manhole. Then, attack them as Hunters and Smokers. They may shoot you, but every so often, you can hurt them until finally they die.

Minigun launcher
Stand behind any mounted minigun on any level, and melee it. You can do this by yourself or get other people to help. Every time your gun hits the minigun, its launching strength will increase. Once you hit it enough, jump on top of it. This will either make you bounce very high or launch you across the map to your death. Bouncing straight up on it allows you to get to unreachable places such as buildings, airplanes, barriers. etc.

Killing two zombies with one bullet
Use a sniper rifle, and zoom in on a group of zombies. When you see a zombie standing behind another one, shoot at the one in the front. The zombie in back will also get shot.

Stronger Hunter
When Hunters are on fire, their pounces and attacks on Survivors do more damage per hit. Use this to your advantage, and deliberately set yourself on fire with fire pits or burning cars, then attack. This allows you to easily down Survivors. Note: This only works for Hunters, not Smokers, Boomers, or Tanks.

Defeating Tanks
To kill a Tank faster, throw a Molotov cocktail, or a gas tank and shoot it. When the Tank is on fire, shoot him with an assault rifle.

Defeating Witches
To kill a Witch faster, throw a Molotov cocktail. When the Witch is on fire, shoot her with an automatic shotgun.

Easy "Akimbo Assassin" achievement
Start a campaign, and shoot a few zombies using only a pistol. Then, select the "Take A Break" option, and allow the NPCs to complete the campaign to get the "Akimbo Assassin" achievement.

Easy "Cr0wnd" achievement
Play on the Easy difficulty. Get a shotgun, and find a Witch. Quickly approach the Witch with your flashlight off. As she rises, quickly shoot her in the head to kill her and get the "Cr0wnd" achievement.

Easy "Dead Stop" achievement
Have a shotgun, and hold LT while running at a Hunter.

Easy "Helping Hand" achievement
While in a safe house, preferably one with a combat shotgun, incapacitate the other three members of your group, then revive them. Doing this counts toward your achievement. It is easier with a combat shotgun because it deals a lot of damage fast. In a single player campaign, make sure all other survivors are in the lower red health area. When you incapacitate one of them, the others will try to revive them. You then need to shoot the others before they revive the incapacitated players or you will not get the points for reviving.

Easy "Man vs. Tank" achievement
Play Dead Air, Runway Finale on the Easy difficulty. Once you have made it to the tanker, refill your ammo, grab a pipe bomb or Molotov cocktail, then answer the radio, and start the gas pump. Once the pump is activated, run around to the right-hand side of the rescue plane, and prepare to fight off the Horde. A benefit of this position is that the Horde cannot come at you from behind. When it is time for the Tank to appear, it will usually spawn on the far side of the plane. Run up to opposite side, then run back and forth without coming around the plane to your side. Your AI teammates will not shoot the Tank when it is on the other side of the plane. By crouching you will be able to easily aim at the lower body from beneath the plane and take it down quickly.

Easy "Unbreakable" achievement
If you have a health pack and are low on health, pause the game, and select the "Take A Break" option. While you are idle, you will be healed. Once you are healed, rejoin the game, and the healing will not count against you. Note: This was done on the Easy difficulty.

Easy "Untouchables" achievement
In No Mercy, Level 5, when you climb to the rooftop, there is a small building with a satellite dish by where the helicopter lands. Climb on top of this building. There is a small staircase on one of the sides leading up to a sealed door (only zombies can open it). Stand above it on the edge, and knock the zombies down as they come up. Eventually one will knock the door down. The Survivors can stand against the back wall in the room, and no infected will come in, including the Tank. Once the helicopter lands, throw a pipe bomb, and quickly run to the helicopter. -From: murderdeathkillr

Easy "What Are You Trying To Prove?" and "Zombicidal Maniac" achievements
There is a glitch that will allow a friend to invite you into the last chapter of any episode (as long as it is from the Dashboard). Completing it from that chapter will still make the game think you have gone through the entire campaign and not just that last chapter. This also works for all other achievements related to completing an entire campaign, such as "Safety First" and "Upset Stomach". Only people invited from the Dashboard will receive the achievements, and not the person inviting.

Achievements
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): Survive a campaign with no Survivors taking damage from Special Infected.
Burn the Witch (10 points): Light a Witch with a Molotov.
Towering Inferno (10 points): Light a Tank with a Molotov.
Spinal Tap (10 points): Kill an Infected with a single blow from behind.
Stomach Upset (20 points): All Survivors complete a campaign without being vomited on.
Brain Salad (15 points): Make 100 headshot kills.
Jump Shot (20 points): Headshot a Hunter while he's leaping.
Mercy Killer (15 points): Survive the No Mercy campaign.
Back 2 Help (20 points): Leave a safe room to save an incapped teammate and bring them back safely.
Toll Collector (15 points): Survive the Death Toll campaign.
Dead Baron (15 points): Survive the Dead Air campaign.
Grim Reaper (15 points): Survive the Blood Harvest campaign.
Ground Cover (30 points): Save another Survivor from a Special Infected while on the ground.
Clean Kill (10 points): Shove a Boomer and then kill him without him splashing on anyone.
Big Drag (20 points): Drag a Survivor 100 feet with your tongue.
Chain Smoker (20 points): Constrict two Survivors in one life as a Smoker.
Barf Bagged (20 points): Cover four Survivors with Boomer bile at once.
Double Jump (20 points): Pounce two different Survivors in one life as a Hunter.
All 4 Dead (30 points): Kill all four Survivors in one life while playing as a Tank.
Dead Wreckening (20 points): Dole out 5000 total Survivor damage as a Special Infected.
Lamb 2 Slaughter (20 points): As an Infected, incap a Survivor who has entered and left a safe room.
The following achievements require the "Crash Course" bonus downloadable content:

Crash-proof (15 points): Survive the Crash Course Campaign.
Quick Power (25 points): Restart the generator within 30 seconds of it shutting off in the Crash Course campaign.
The Littlest Genocide (25 points): Kill 5,359 Infected in the Crash Course campaign.
Smash Hit (25 points): Win a Versus campaign of Crash Course.
Truck Stop (35 points): Your team wipes all Survivors after the escape vehicle has opened in Crash Course.
20 Car Pile-up (20 points): As a Tank hit 20 Survivors with a car in the Crash Course campaign.
Jumpin' Jack Smash (25 points): Pounce a Survivor for 25 points of damage in the Crash Course campaign.
Slippery Pull (25 points): Smoker pull a bile-covered Survivor until you hold him during Crash Course.
Tank Stumble (20 points): Stun a Tank with an explosion in the Crash Course campaign.
Wipefest (35 points): Your team incapacitates three Survivors within five seconds in the Crash Course campaign