Mini Nuke (Fallout 3)

The Mini Nuke is a tactical nuclear warhead designed for use in a shoulder-launched missile weapon. It is used as ammunition for the Fat Man and its unique variants.

Mini-Nukes are the single most expensive piece of ammunition, ranging from 250 to 400 caps a piece to buy, depending on your Barter level. They are also the single most powerful weapon in the game, doing many times more damage than any other weapon even at low Big Guns levels. These are also great decoration for your house or Tenpenny Tower Suite. This is clearly another reference to the Nagasaki Fat Man because it closely resembles the look of it, but smaller.

Hand grenade
A hand grenade version of Mini Nuke also exists, although it can be even more dangerous to the user than the Fat Man version.

Fallout 3

 * Fat Man
 * Experimental MIRV
 * Liberty Prime (a larger version, but less effective)
 * VB-02 Vertibird

Available for Free
There are 59 total, in Fallout 3. Another is included with the Operation Anchorage DLC package and is at the bottom of this list, making for 60 total.

Available in Other Ways

 * One can be obtained by investing twice in Lucky Harith (see Canterbury Commons).
 * One is given to you by Donovan if you experience the random encounter with him in the Wasteland.
 * Flak and Shrapnel in Rivet City frequently restock with Mini Nukes, to the extent that one can add up to three of them with every visit to sell loot.
 * Moira Brown will give you one upon completion of the The Wasteland Survival Guide (quest). She tells upon giving you it that she was going to try to make a well with it, but that the mini nuke only made her nervous.
 * Herbert Dashwood's safe contains one. You can get the key through one of three ways: Kill him, pick his pocket, or reveal the fate of his lost ghoul friend, Argyle.
 * Agatha (woman from the quest Agatha's Song) will give you the key to her ammunition chest, which contains a mini nuke and a lot of other ammunition, if you complete her speech challenge, or alternately; kill her.
 * One can be obtained along with a Fatman during a random encounter during which a dead scavenger has them on his corpse.
 * You may also get a mini nuke by killing Super Mutant Overlords or Behemoths. (Confirmed)