Thor's Hammer pendants were very common during the Viking Age.  Thor was one of the most popular of the gods.  Many stories tell of one giant or another trying to bring down the gods and Thor ultimately stops him.  This makes Thor the protector of the gods and mortals alike.  His most prized possession was his mighty hammer Mjolnir.  When thrown at a target Mjolnir would magically return to him.  It is Thor's powerful hammer that becomes the symbol of Thor and is worn as a symbol of protection.