Bile Bomb

The Bile bomb is a new weapon appearing in Left 4 Dead 2. The weapon was first mentioned in the new achievement list here. Not much is currently known about this weapon, except that it contains Boomer bile and based on the achievement Septic Tank, looks like the Infected summoned by the bile will attack whatever is covering it.

Once you kill one of the Hazmat Infected, they should drop one of these containers full of bile. There's an achievement to pick up 10 of these containers after killing the Hazmat Infected. It will most likely use the same inventory slot as the pipe bomb and the molotov.

It has been speculated that the bile bomb will have similar functions to the pipe bomb, as Boomer bile calls the horde. The bomb could do the same and the horde summoned by the Survivor will attack wherever or whatever the bomb hits, including other Infected. This is re-enforced by the achievement gained by covering a Tank in such bile, which would probably cause other Infected to attack it.

As seen in one of the newest videos featuring Hard Rain, the bile bomb is infact used to call a horde of regular infected to attack special infected. It uses up the slot for Pipe bomb and Molotov