Vikings in Norway grew hemp.
Examination of material excavated from a Viking farm in southern Norway uncovered hemp pollen dating from 650 to 800 AD. Hemp is Cannabis sativa, a subspecies of which, Cannabis sativa indica, is marijuana. Industrial hemp such as what the Vikings grew can't get you high because it contains almost no THC, the active ingredient in marijuana.
Instead, hemp can be used for clothing and rope, as well as numerous other products. While the archaeologists stressed that they found no evidence that the Vikings grew marijuana, I have a hard time beliving they didn't have a little patch set aside for those long winter nights. The sagas would have sounded pretty cool while high on a mixture of pot and mead.
They couldn't have smoked too much, though, otherwise they wouldn't have made all those voyages of conquest and discovery. They'd have just stayed home eating Doritos and watching TV instead.
Photo of industrial hemp (not the Grateful Dead kind) courtesy Evelyn Simak.