Flashlight

The Flashlight is an item that is always in the Survivors' inventory and is attached to every weapon the player can obtain. You cannot drop it, but you can turn it on and off, which makes a clicking sound. It allows the player to see in dark areas when exploring for items and weapons. It also allows the player to scout out such areas for groups of Common Infected, and spot Special Infected much easier. Also, the flashlight is attached in a fixed point on your weapon which means that when using the melee attack, changing weapon and reloading, you lose the benefit of the light as your weapon moves and changes angles.

The flashlight is a sure-fire way to disturb the Witch, as demonstrated in the introduction cutscene. The Survivors will often warn each other when they believe they hear a Witch, telling them to turn their flashlights off. Shining the light on her will disturb her more than standing near her, so you should aim your crosshair away from her, or turn it off altogether. Also, idle Common Infected are disturbed faster when you shine your flashlight on them.

Behind the Scenes
Originally, weapons did not have flashlights on them. The light came from the player's chest, like in the Half-Life series. Flashlights were attached to the guns to make it so that the player wouldn't have a light source when either reloading or using melee.

The old flashlight behavior is still in the final game; turn on the flashlight when holding a grenade, Medkit or Pills to see it.

Trivia

 * You can still somehow use the flashlight even when wielding items such as pills, and gas cans, even when they do not have one attached to them.
 * Even though the common tactic is to switch off your flashlight when a Witch is nearby, it is usually unnecessary as the Witch is only disturbed by the light when you aim it directly at her.
 * While you cannot see the light made by another Survivor's flashlight, you can tell if they have their light on or off, by looking at the bottom of their gun. If the flashlight is on, there will be a cone of light there.
 * When approaching a Common Infected to melee in the back, be sure to turn the flashlight off so the Infected usually won't notice you coming. They react slowly even with your flashlight on, as long as you don't shine it into their eyes (or what's left of them) they shouldn't notice you right away.
 * Turning on your flashlight while you're aiming the minigun will cause your flashlight to rapidly turn on and off continuously until you dismount.