Fat Man (Fallout 3)

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

Appearances
The Fatman 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 Fatman 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 Fatman can also be found in Evergreen Mills, and another one is present at the Talon Company fort.

EDIT: A fatman can be aquired 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.

Via use of the console, a MIRV version of the Fatman can be gained, which fires 8 mini nukes at once. It is unknown whether this can be obtained via normal play.