There is no shortage of free wargames rules for the Colonial period. A good one is the Two Page Skirmish rules from Steve Winter.
Miniatures Rules
Combat WWII 15mm Rules
Combat WWII is a set of free wargames rules for 15mm WWII action.
De Bellis Velitum
De Bellis Velitum is a set of free wargames rules from the legendary Phil Barker that features:
two small forces, each consisting of 1 to 4 sub-units, each in turn of 4 to 12 singly based 25mm figures, totalling up to 24 army points (AP) and controlled by a single player. Figures vary in cost between 6 AP and 2 AP. One figure of the most expensive mounted type or most expensive foot type present is nominated as the force’s commander and one figure of each group is nominated as its leader.
Taktika Miniatures Rules
Taktika is a set of free wargames rules for science fiction battles.
Black Hat, White Hat
Black Hat, White Hat is a set of free wargames rules for Wild West skirmishes.