Viking Raid is a set of free wargames rules that uses playing cards for action resolution.
Here’s a nice video tutorial on making a Viking longboat.
Aegir’s Fire is a set of rules for a Viking campaign. Author Jim Wallman writes:
In this game, you are a minor Viking village leader (think up a suitable name). You have managed to stretch the village�s resources to the point of building a longship, to the latest and most fashionable design. You will be able to equip and man the ship from your village initially, and dependent upon success or otherwise in your first voyages, you will be able to recruit more experienced fighters from outside the village. Success is measured in two ways, status and treasure.
East Street games offers Valhalla, a set of free wargames rules for Dark Age skirmishes. The rules are free for the downloading, or for purchase in print. The website has an online system for generating warbands. Interesting stuff.
Saxons and Vikings is a set of four page simple rules for Dark Ages battles