From Shieldwall Games, Skjaldborg is a set of free wargames skirmish rules for ancients through medievals. They look like they’d be best for dark ages — and especially Vikings! It is very professionally presented in a pdf file and has some interesting rules mechanisms. My favorite part is how all of the charts work the same. You start in a base column and then shift left or right depending upon the situation. The modifiers are presented below the appropriate chart, with little arrows telling you which way to shift.
I’ve saved this one to my hard drive. It’s a must-try in my Viking project.