Panzer Gruppe is a set of free wargames rules for corps/army sized battles in World War II.
Into the Dark is a set of free wargames rules by Dan Abbott for mass combat in the Medieval period. Designed for 25mm to 30mm figures, the rules seem designed to allow players to easily manage large numbers of figures. In combat, 1d6 is thrown for every four figures in combat. Modifiers are kept to a minimum.
More from Dan:
I’m aware that there are many rule sets on the market designed to deal with medieval armies. However most of these sets are meant to deal with skirmish fighting. Even some of the better known rules for armies are really just skirmish rules done on a giant scale. The rules you see before you are designed for battles between armies, similar to those found in history or heroic literature. Large scale battles can be fought to a conclusion in a reasonable amount of time.
These rules were originally designed for 25mm/30mm figures, although they can easily be used for 15mm by dividing all ranges in half, or by using centimetres instead of inches. Don’t be put off by the size of the rules, use only the basic rules until you are comfortable with play, then add whatever optional rules you like. The section on game preparation takes up a lot of room, but is only needed when first creating your army.