Fat Man (Fallout 3)

The Fat Man is a handheld tactical nuclear catapult, found in Fallout 3.

Appearances
The Fat Man appears only in Fallout 3, and can be found in the Rivet City market area. Because of its nuclear payload, it is by far the strongest V.A.T.S. enabled weapon in the game, perfect for taking out multiple enemies or heavily armored units - and in normal use, you'll be encountering both. The Fat Man has an estimated range of 150+ yards and will fall if shot in the air; be aware that the blast zone will be irradiated. You can find this gun right outside of GNR, as well, during the "Following in His Footsteps" quest by picking it up from a dead Brotherhood soldier.

A Fat Man can also be found in Evergreen Mills, and another one is present at the Talon Company fort.

EDIT: A Fat Man can be acquired very early in the game. One of the paladins has one when you first encounter the extremely large super mutant. If the mutant kills him you can pick up the weapon with 7-8 ammo.

There is a MIRV version of the Fat Man that can fire 8 nukes at once, this can be found in the National Guard Depot.