Pipe Bomb

The Pipe bomb is a powerful homemade explosive that doubles as a diversionary device. The design is based off a stereotypical pipe bomb, but with the gutted remains of a smoke detector taped to it. When it is thrown, it emits a loud beeping sound and flashing red light, so any Common Infected not already in melee range of a Survivor will be attracted to it. When the fuse is depleted, it will detonate and kill all nearby Infected. The pipe bomb may also damage Survivors caught in the explosion radius. The pipe bomb can be an extremely useful weapon in many situations. You can use it to defend victims of the Boomer's bile, since the Common Infected summoned by the bile will chase the pipe bomb in preference. It can be used for clearing out an area of Common Infected, for conserving ammunition, and for crippling a horde, particularly when being attacked from multiple directions or during a finale.

Tactics

 * Hold on to pipe bombs until attacked by a large horde. The effectiveness of the pipe bomb is proportional to the number of Infected nearby.
 * If there are Common Infected who are visibly unable to reach the pipe bomb before its detonation, shoot them as they are running so that there are fewer left over to kill. Large groups can be thinned out quickly this way. A pipe bomb can be combined with a molotov cocktail for a similar effect.
 * Throwing multiple pipe bombs at once is inefficient, as Common Infected tend to run towards the first one thrown. Let your teammates know when you are going to throw a pipe bomb.
 * During a finale or Panic Event, throw the pipe bomb when most of the horde are on their feet. If a Common Infected is climbing when a pipe bomb is thrown, then they will finish climbing before they run towards the pipe bomb and therefore they may not reach it in time.
 * The Common Infected who are not killed by the explosion are stunned, and therefore very vulnerable to attack.
 * The kill radius may vary with the difficulty level.
 * Special Infected, in campaign or single-player, will not be attracted by the pipe bomb. However Witches can be startled if the explosion occurs too close to her personal space. Molotovs or gas cans are preferable when faced with Special Infected.
 * In Versus Mode, the distinct beep of the pipe bomb will make them almost useless against Infected players, as they can easily avoid them, or even run right past them.
 * During a finale, it is always good for at least one player to hold onto a pipe bomb until the rescue vehicle arrives, then to use it. This will distract the horde long enough to get in the rescue vehicle.
 * If there are Infected near you, or come too close to you while running towards the pipe bomb, they will forget about it and start attacking you. Use melee attack to push them away which will cause them to disengage, and chase after the pipe bomb.
 * If timed perfectly, a pipe bomb will stun an oncoming Tank (much like when you melee a Common Infected). The Tank will become stunned and knocked back a little, long enough for you to fill him with lead.

Behind the Scenes
According to the game's Developer Commentary, the pipe bomb did not have the flashing light when it was originally conceived; instead, it served as a conventional grenade. There were some issues with this however: rushing Infected would ignore the grenade, and often run past it, making the pipe bomb useless unless perfectly timed. Another, worse, problem was when the Infected not only ignored the grenade, but kicked it back toward the thrower (not in a literal sense, but it would bounce off of them). Valve fixed the problem by making the pipe bomb attract Common Infected, and added the oxygen tank to perform the role of the earlier pipe bomb.

A grenade icon hints that the pipe bomb was actually going to be a regular grenade at one time. This makes sense, as the pipe bomb is based of of the HE Grenade in Counter-Strike (see below).

Trivia

 * If you want some laughs, throw your pipe bomb at a Infected who isn't attacking you and it will "dink" off and the Infected will twitch a little.
 * Also, the pipe bomb is lit when you are holding it, making it seem as if it would explode in your hands.
 * Strangely, when you toss a pipe bomb, all the Common Infected charge after the bomb. However, when you set off a car alarm, they seem to be interested in you, not the car. The reason for this is unknown, but since the Infected are sensible to loud noises, it might be possible that the pipe bomb emits a higher-pitched sound, therefore being more attractive than the Survivors, and while the car alarm attracts the horde, the possibly lower-pitched sound is less attractive, so upon spotting the Survivors, the Infected are more interested in them than the car alarm.
 * The pipe bomb is based off of Counter-Strike's HE Grenade. It's "price" in the pipe bomb's weapon file is the same as the HE Grenade's, the pipe bomb's sounds are in a folder called "hegrenade" and there is an early grenade icon in the game's texture files. This grenade looks exactly like the Counter-Strike: Source HE Grenade.
 * In Left 4 Dead 2, the explosion graphics have been greatly improved. Instead of all the Infected disappearing into a haze of blood, only the inner Infected receive this fate, whilst all the rest are blown away spectacularly and rather amusingly.
 * You can throw pipe bombs off a ledge or a high place, resulting in all of the Common Infected following it down inevitably leading to their death.