Typically used in Pagan traditions with a Norse background, such as Asatru, this symbol (also called Mjolnir) represents the power of Thor over lightning and thunder. The early Pagan Norsemen wore the Hammer as an amulet of protection long after Christianity had moved into their world, and it is still worn today, both by Asatruar and others of Norse heritage.
Mjolnir was a handy tool to have around, because it always returned to whomever had thrown it. Interestingly, in some legends Mjolnir is portrayed not as a hammer, but as an axe or club. In Snorri Sturlson's prose edda, it is said that Thor could use Mjolnir "to strike as firmly as he wanted, whatever his aim, and the hammer would never fail... if he threw it at something, it would never miss and never fly so far from his hand that it would not find its way back."
Images of Mjolnir were used throughout the Scandinavian countries. It was often found replicated at Blóts and at other rituals and ceremonies like weddings, funerals, or baptisms. In areas of Sweden, Denmark, and Norway, small wearable versions of this symbol have been unearthed in graves and burial cairns. Interestingly, the shape of the hammer seems to vary a bit by region -- in Sweden and Norway, Mjolnir is portrayed as rather t-shaped. Its Icelandic counterpart is more crosslike, and examples found in Finland have a long, curved design across the bottom brace of the hammer. In contemporary Pagan religions, this symbol can be invoked to protect and defend.
Thor and his mighty hammer appear in a number of aspects of pop culture as well. In the Marvel comic book and movie series, Mjolnir serves as an important plot device when Thor finds himself stranded on Earth. Thor and Mjolnir also appear in Neil Gaiman's Sandman graphic novels, and the television series Stargate SG-1 includes the Asgard race, whose spaceships are shaped like Mjolnir.