Chainsaw

The Chainsaw is a new close combat weapon introduced in Left 4 Dead 2. As seen in the beta version footage, using this weapon against the Infected can bring them the worst injuries, sometimes cutting them in half.

It is stored in the same slot as the pistol, making yet another tactical choice in the game. It has unlimited uses, meaning that it can be very useful when the Survivors run out of ammo in the middle of a combat situation where it's too dangerous to use the pistols, such as hordes with a Hunter hiding somewhere among the Common Infected. Like the other close combat weapons, it can kill every Common Infected in one hit, but it has its disadvantages, as the Survivors cannot use any other weapons while carrying a close combat weapon. This makes it extremely dangerous to attempt combat with Special Infected, as it will take couple of seconds to drop the weapon and assist a pinned Survivor.

The chainsaw is not really meant to be used against Special Infected; using it on a Boomer would spread bile over the Survivors, and using it against a Spitter would leave you right in the middle of the goo. Against a Witch or Tank, it would do little to no damage.