Vikings did a lot of navigating in their line of work. Those who didn't know how to use the sun to get exactly where they wanted to go would end up razing only sandcastles, pillaging only bird's nests, and setting fire only to their own beards. So when cloudy, foggy, gray days caused these intrepid seamen to tear the horns off their helmets in frustration, they brought in a pinch hitter. The sunstone was a magical rock that the Vikings used to find where the sun was, even on severely overcast days.
Most people believe that the sunstone was a myth. The Vikings didn't have any special stone, they were just very good navigators. Now, some scientists believe that the son stone did exist, and that it did work. The vikings weren't using magic; they were using polarized light.
Polarized light, light waves which are all oriented the same direction, is not as exotic as it sounds. The atmosphere polarizes sunlight, to a certain degree. Sunlight is polarized along a line tangential to concentric circles around the sun.
When polarized light is forced through a lattice, like when it shines through a crystal, light waves oriented in the wrong direction are choked off. Those that are oriented in the correct direction are let through. When the vikings looked at the sky through the crystal, and rotated the crystal around, the crystal would flicker lighter or darker, depending on how much light was let through. A few tries, with the crystal in a few different places, and the marauders would be able to drunkenly steer their ships in the right direction and sack something. At least, in theory.
There's much debate about whether or not sunstones were used. Some people contend that crystals would be no more accurate than eyes on a cloudy day, and far less accurate than eyes on a sunny one. Others say that, even when it's impossible for the eye to tell exactly where the sun in coming from, the crystals still flicker tellingly. So far, though, sunstones remain only in legend. If you want to pile into a wooden boat, push off to sea, and navigate by crystal; pack a lunch, and never tell anyone where you got the idea to do it.