The Megaton hammer is an item found in Nintendo's game The Legend of Zelda: Ocarina of Time. Our young hero Link obtains this item in the Fire Temple, and uses it to smack round that level's boss, a big ol' fire dragon by the name of Volvagia.
Getting ahold of the Megaton hammer is quite a pain. Link comes upon a large pit, along the edge of which runs a narrow stairway. At the top of this stairway is a chest protected by flame. A switch at the bottom of the stairs will turn off the fire, but only for a short time. So Link basically has to run up a very thin, curvy stairwell within a fixed time to get the hammer. If he falls, he drops several hundred feet (thankfully, Link is very hardy, and this will only leave a few bruises). It often takes two or three tries to get this right -- either you go to fast and fall off, or you are too cautious, and don't reach the chest in time.
In addition to committing assault and battery on fire dragons, the hammer is also useful for smashing down rusted switches and cracking open certain boulders which Link's explosives will not harm. It can also be used as a weapon against other foes, though its short reach makes it less than optimal. In addition, unlike the Master Sword, you can't use the shield at the same time as the hammer. However, unless you get the Biggoron's Sword (also sometimes called the Goron's Knife), a massive long sword, you will be forced to use the hammer in the final fight, where Link loses the Master Sword. It is also pretty useful against main bosses, as Zelda bosses tend to be of the "stun them and hit them" style. Since they're not moving anyway, you can pick and choose your method of breaking their heads.
By default, an attack with the hammer will only strike straight down. However, with some practice, you can get Link to swing the hammer from side to side, which is usually somewhat more effective if you're fighting standard issue bad guys with it.
This item does not appear in Majora's Mask. It first (?) appeared in A Link To The Past.