In sagas recording Viking navigational stories, it's written that they used stones that could point to the sun in any conditions. A study, directed by Guy Ropars (University of Rennes) and published by the Proceedings of the Royal Society, shows that they may have used a mineral called iceland spar, a form of calcite. The passage of sunlight polarises it, which means that light from the sky points towards the sun, in certain conditions.
Source : the Economist, Discovery News.
MAJ (19/11/2011) : someone I know reacted to this post by saying that sometimes we have the feeling that with our technology we can do more things that we ever did, but often it's just that we have forgotten how we did it before. Moreover, even if with technology we are able to do things that were forgotten, just look at the cost. Vikings could navigate with a stone. We need GPS. What goes with GPS ? A manufactory, powered by an electrical station. A satellite in the sky, that needs to be launched by a rocket, built by another manufactory, etc.