Gas can

The Gas can, also known as fuel or petrol cans; can be found throughout the game. They can be picked up with the Use key (default: "E" for PC; "X" for Xbox 360) and release flames similar to a molotov cocktail when shot. A player can not shoot while holding a gas can, but can still melee. There is no inventory space for this weapon, so the only method of transporting it is to pick it up and carry it. These cans are usually found around Crescendo Events and finales. They are used to set up traps and provide a strong defense during large horde attacks. Strangely, the gas can makes the same sound of glass shattering that the molotov does when ignited. Also sometimes when a Survivor finds one it will yell "Molotov here!" as if they had just found a molotov cocktail.

Commonly used for defending against large attacks such as hordes and for survival maps, people will often attempt to store them and dispense them. This can spell "disaster" for your team, as if they are mistakenly shot, you must flee from your camping spot and become vulnerable.

A useful tactic with gas cans is to spread them in an arc in front of you, as flames from a gas can ignite unlit ones, causing a large chain reaction that helps with horde attacks and finales.

In Left 4 Dead 2, the new mode, Scavenge Mode, uses gas cans as a non-offensive purpose. Yellow gas cans are scattered across the level. These gas cans must be taken back to a generator to refuel it, instead of shooting them. Gas cans can still be shot, but it isn't recommended or wise, as they are valuable. Unlike other modes, certain Infected like the Spitter can ignite these gas cans, to not only prevent them from being poured, but to damage Survivors.

Tactics

 * The gas can has a wider area of effect than the molotov cocktail, making it more effective for large choke points and wide open spaces.
 * If placed before a showdown or event, the gas tank can be significantly less dangerous than thrown weapons. Due to lag or confusion, a molotov cocktail and pipe bomb can land in close proximity to the Survivors, making them a detriment and sometimes forcing the Survivors to abandon their preferred choke point. The gas can provides a safe weapon that can be set off from a distance with no risk of accidental targeting.
 * The gas tank can be carried from level to level if it is brought into the safe room. If you find a gas tank near the end of the level, it might be more valuable to save it for the next level than waste it (campaign and single player only, not Versus).
 * While it is often better to drop the gas can for hordes, small groups and single common Infected can be killed with the melee attack of the gas tank. This means it can often be carried much further than most people think.
 * The fire from the gas can doesn't normally burn in water, but has been known to burn in a few bodies of water.
 * The gas tank cannot be damaged while being carried, so don't worry about friendly-fire killing everyone. But note, as soon as the tank is dropped, it is damageable again, leading to events just as bad as being shot while you're carrying it.
 * If a gas can catches on fire, it may be possible that you can pick it up and drop it without it blowing up.
 * You can use the gas can to get the "Burn the Witch" and "Towering Inferno" achievements, despite the fact both achievements say you need to use a molotov.

Survivor Tactics

 * Don't stand near a gas can when you set it off.
 * When faced with a Crescendo Event or a finale, it's generally best to set up first, and this includes searching the area for gas cans and propane tanks, or other explosives. Try to place the gas cans in choke points, or in areas where the Infected usually come charging in in massive groups. Once there is a large group of Infected over it, or relatively near it, set it off. The flames will effectively thin out most, if not all, of the zombies in that rush.
 * Gas cans can make decent substitutes for a molotov when facing a Tank. Set one down if you know you're going to be facing one, and try to back-pedal over it. Shoot it and set it off, lighting the igniting the Tank as runs through the flames.
 * If a gas can is thrown, by pressing the "fire" key while holding it, rather than dropped, it bounces and won't roll in a straight line, meaning it may not reach its intended destination. Therefore, it is usually best to drop it right on the spot you want it to be in by switching to any inventory item.
 * Gas cans are handy for covering your back in a Panic Event.
 * Be careful when using them: before they burn out, the fire will spread about 2 feet, so being right next to it is not a good idea.
 * Don't shoot the gas can if you are incapacitated, this makes more problems for your teammates when they try and revive you.

Infected Tactics

 * Gas cans, in addition to propane tanks and oxygen tanks, give a clear signal of where the Survivors expect Infected to come from. You can use this to your advantage by spawning or moving to areas that are in the opposite directions of these weapons.
 * Another strategy is to light yourself on fire as the Hunter and pounce the Survivor holding the gas can. The pounced Survivor will drop the gas can, which will ignite on contact with your flaming body, dealing even more damage to the pounced Survivor and making it extremely difficult for other Survivors to melee you off or save the other Survivor if they are incapacitated.
 * If you're playing as a Smoker, one strategy is to position yourself on one side of the flames and drag a Survivor through it — especially if you can keep the Survivor in the flame (great if it's a set flame, such as some of the flames on The Subway), without being burned yourself.

Trivia
The Gas can's model is from Half-Life 2, but is reskinned to fit Left 4 Dead.