Mjolnir

Mjölnir (also known as Thor's Mystical Hammer or simply The Mystical Hammer) is the primary weapon of Thor, the Asgardian God of Thunder. It is an enchanted hammer of tremendous power that grants Thor the ability to fly and summon lightning against foes. 'It is Thor's main weapon in the 2012 action film The Avengers.

History
Mjolnir was forged in the heart of a dying star by dwarves. With it, Thor can harness the power of lightning bolts, and fly. Only those individuals that are worthy of the power of Thor can lift the Mjolnir, as Hulk discovered he couldn't lift it; this spell was placed upon it by Odin himself when he punished Thor with banishment to teach him a lesson. On the side of the hammer, there is an inscription engraved "Whosoever holds this hammer, if he be worthy, shall possess the power of Thor..."

Thus far, it has been lifted by:
 * Thor
 * Odin
 * Beta Ray Bill

The Avengers
Thor uses Mjolnir against Captain America and Iron Man and it proves unable to break Captain America's vibranium shield. When the Hulk rampages through the helicarrier, Thor seeks to defeat him. During this fight, the Hulk attempts to lift Mjolnir but is unworthy, therefore making Mjolnir one of the only objects the Hulk cannot lift. Thor uses Mjolnir to great effect during the Chitauri invasion.

Powers and Abilities
Mjolnir was forged from the heart of a dying star. Thor's mystical hammer Mjolnir, which resembles a mallet rather than a war hammer, has a number of elemental powers. It has been stated be Odin himself that Mjolnir's power has no equal. Mjolnir itself is extremely durable like the Earth metal Adamantium, and combined with the various enchantments placed upon it by Odin, it is, for all intents and purposes, indestructible. It has deflected three blast from the Destroyer (which was able to disintegrate anything that it hit) and return the blast back into the Detroyer. Thor often uses the hammer as a physical weapon, with almost nothing being capable of withstanding a hammer blow or throw. A few examples of the abilities Thor has shown with Mjolnir:
 * Weather Control: Wielding Mjonir grants Thor the ability to control the base elements of a storm. It can control the elements and can create giant raging electrical storms complete with thunder, lightning, hurricane-force winds, tornadoes, tidal waves, earthquakes and torrential rains at a moment's notice. He summoned a class 5 tornado to battle the Detroyer, a lightning bolt strong enough to destroy a large piece of landscape in Johtunheim, a lightning bolt to knock down Loki on the Bifrost bridge, and control the elements. He is remembered in myth and legends by the mortals of Midgard as the Norse Thunder God who used Mjolnir to summon rain, wind, thunder, lightning and more.
 * Worthiness Enchantment: After Thor disobeyed Odin and nearly caused a war between Asgard and Johtumheim, Odin banishes Thor to Earth without his powers and places an enchantment on the hammer Mjolnir. "Who so ever holds this hammer, if he be worthy, shall possess the power of Thor." This enchantment surrounding Mjolnir prevents it from being wielded by anyone save those who have been found worthy. When Thor sacrificed himself to protect the New Mexico town and it's inhabitants from The Destroyer he proved himself worthy to wield Mjolnir and gained full use of his powers. He also used it to trap Loki on the Bifrost, as those who aren't worthy cannot lift it. Thor used this power to distract the Hulk in their battle, who became so focused on trying to lift the hammer that Thor was able climb onto his back and attempt to choke him into submission.
 * Mystical Link: Mjolnir obeys Thor’s commands as though it were alive. Mjolnir went to Thor when it was in a crater from miles away, it came to him in seconds. Mjolnir will return to Thor when he throws it at an opponent.
 * Flight: Thor is capable of hurling Mjolnir with great force and, by holding onto the leather thong, is capable of flying through the air at tremendous speeds. How fast is not specified, but he was able to fly in Johtumheim to kill a Frost Monster, fly in the middle of a Class 5 tornado, fly from the small New Mexico town to the closest Bifrost site, fly from Heimdall's Observatory to the main hall of Asgard in seconds, and later from Asgard back to Heimdall's Observatory.
 * Energy Projection: With Mjolnir, Thor can project powerful mystical blasts of energy. He can even channel energies for stronger energy attacks. With Mjolnir, Odin projects a powerful mystical blast of energy using it to channel an energy blast through it, taking away all of Thor's superhuman powers and destroys what was left of Thor's armor after stripping much of it off of him and throwing him into the Bifrost.

Trivia

 * In the comic books, Captain America is able to lift Mjolnir. It has also been used by Zero-G (Alex Power), the leader of the young superhero team, Power Pack
 * The only known beings in the Marvel Cinematic Universe who have been known to lift the hammer are Thor and Odin. Those who have attempted and failed are Loki, the Hulk and various inhabitants of the New Mexico town in which Thor was exiled.