The inscription on Mjolnir clearly reads, "Whosoever holds this hammer, if he be worthy, shall possess the power of Thor," and a number of heroes - including Captain America and Beta Ray Bill - have done just that over the years. However, one of the most surprising and unexpected wielders has to be Superman. After besting the God of Thunder in combat during one of the rare crossovers between Marvel and DC Comics which paired up the Avengers and Justice League, Odin allowed the Man of Steel to use Mjolnor during a pivotal moment in battle. After all was said and done, Superman found himself unable to budge it!
