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 will emit a loud beeping sound and flashing red light for 5-7 seconds, drawing towards it any Common Infected not already in melee range of a Survivor. 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. Pipe bombs 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.
 * 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 Infected are near you, or come too close to you while running towards the pipe bomb, they will forget about it and start attacking you. Use your 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.

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 and the Pipe bomb's sounds are in a folder called "hegrenade".

It is likely that the Pipe bomb was a regular grenade at one time. Hidden in the game's files is a very early grenade icon for the HUD; this grenade looks exactly like the Counter-Strike: Source HE Grenade. This, combined with the earlier info about the Pipe bomb being based off of the HE grenade, means that the Pipe bomb was likely a regular grenade at one time.

One of the proposed uses of the pipe bomb was for it to be an anti-Tank weapon. This ability is discussed in the Developer Commentary and can be seen in this video dated August 27th, 2008. While the video doesn't show the pipe bomb killing the Tank, it portrays it as useful against one.

Trivia

 * In Left 4 Dead 2, the explosion graphics have been altered. Instead of all the Infected disappearing into a haze of blood, only the inner Infected receive this fate, whilst all the rest are launched into the air as ragdolls.
 * 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.
 * In Left 4 Dead 2, Hard Rain's Uncommon Common infected ignore pipe-bombs.