Molotov Cocktail

The Molotov cocktail is a thrown weapon that engulfs an area with flames for a short time. The area of effect is quite large and spreads over time before self-extinguishing. The fire will burn all Infected that come into contact with it. While Common Infected cannot attack after being ignited and die in seconds, Special Infected can withstand the fire until their health is burned away. The Molotov cocktail is dangerous in close combat because the splash damage and flames may cause you and nearby teammates to lose health.

Tactics

 * If a Boomer is around a corner where the wall is impenetrable by bullets, and you do not wish to risk being covered in bile by attempting to land a well-angled shot, throw a Molotov around the corner. The flames will spread, so it is not required to hit him directly in order to kill him. However, the Boomer becomes a ticking time bomb and may run toward you and your teammates in order to splash them before he dies; if this does happen, it would be a good idea to run up to him and knock him back with a melee attack before he explodes.
 * Do not hit teammates. If they are incapacitated inside a fire, it may be difficult to revive them.
 * Where the pipe bomb is less effective (e.g. against Special Infected) due to its fuse, the Molotov shines. Setting a Witch on fire will gradually burn her health away and make her move extremely slowly, making her easier to kill. A burning Tank will survive for 30 seconds on normal, 40 on advanced, and 45 on expert. Only in campaign mode, setting the Tank on fire will increase his running speed, since he doesn't have much time to attack the Survivors.
 * Though it may seem like a good idea to light a Tank up in single player, the end result is usually the same: three dead bots and a lonely Survivor. Bots tend to ignore fire altogether, wading through it like a very hot day in the park, and will continuously shoot at the Tank, burning or not. A better strategy is to focus your gunfire on the beast, even on Expert. Oftentimes, only one or two Bots will be dead, and you can continue on to the safehouse.
 * No Infected of any kind know better than to avoid fire and will gladly cross it to reach you. The crux of the Molotov strategy is to let them. If you know where a Horde is coming from, throw the Molotov at a choke point between the Horde and your teammates. The flaming barrier will burn all Common Infected as they try to cross it. Try to throw the Molotov just as they reach the choke point in order to increase the likelihood of burning the entire Horde before the flames expire. Note that sometimes they will try to find a way around the fire, but it is not likely. Infected tend to want to get from point A to point B (i.e a straight line) unless a static object is in the way, like a big tree trunk.
 * Obviously, fire does not burn in water, though alcohol fires can burn in water.
 * There is at least one exception to this: Death Toll's first Crescendo Event in the storm drains. A Molotov thrown into the shallow water will burn as normal.
 * Once lit, Special Infected do not self-extinguish. Unless the map has water on it (e.g. Death Toll Finale). They will burn until they die. This applies for Tanks and Witches, too.
 * In Versus, if a player is hanging and being attacked by a Smoker out of reach from normal weapons, consider lobbing a Molotov to release them.
 * Since the Molotov will not set off car alarms, a common strategy is to lob a Molotov at a car alarm to kill any Smokers that would try to pull you towards the car (which will set off the car alarm).
 * When on the Rooftop Finale in the No Mercy campaign, when you get to the ladder, you can, rather than just lobbing a Molotov onto the roof, climb the ladder and check where the Infected are. Usually there's a Smoker south-east of your position. Lobbing a Molotov there from the elevator shaft should get rid of a Smoker, thus leaving a Boomer and two to three Hunters on the roof to deal with. This takes a lot of practice and should be practiced on in single-player with cheats on.

Infected

 * Though it will eventually kill it, the Hunter does twice the normal damage when it is on fire. Note that after the recent update, the bonus fire damage is only gained by lighting a Hunter on fire using a molotov. The Hunter doesn't deal as much damage as it used to after the update as well.
 * As a Boomer, you can set yourself on fire to become a ticking time bomb. Run into Survivors to forcefully explode, since the Survivors can't safely push you away and shoot you without the risk of being exploded on. However, Boomers don't have very much health, so don't set yourself on fire too early.
 * If you're playing as a Smoker, a great tactic is to drag a Survivor through the flames, causing massive damage and preventing others from rescuing them. Take care as they can kill you with their weapons; flame doesn't burn bullets.

Left 4 Dead 2
In Left 4 Dead 2, during the Parish campaign, using a molotov or incendiary ammo on the Infected that are wearing HazMat suits will have no effect.

Behind the Scenes
An early screenshot shows that the Molotov was grey and had a different-shaped top before becoming the final design.

Trivia

 * The molotov also appeared in the Half-Life 2 beta.
 * You can hold a burning molotov for as long as you want, and it will not explode or catch fire, even though it already is on fire. Technically this is because the bottle has to break and spread the fuel inside to cause the primary burning. The pipe bomb and its lit fuse can also be held indefinitely.