These aren’t exactly free wargames rules, but if you already own a set of Warhammer 40K, this is worth checking out. Jervis Johnson has provided this page of colonial amendments to Warhammer 40K. These also would be a good way to introduce fantasy and science fiction wargamers to historical miniatures games.
Rules: Colonial
When Empires Clash Colonial Wargames Rules
When Empires Clash is a set of free wargames rules for fighting out large scale battles in the 19th Century colonial era.
Redcoats and Natives Colonial Rules
Bob Cordery offers Redcoats and Natives, a set of free wargames rules for the colonial period.
New Zealand Colonial Wars DBA Variant
Stephen Thomas offers a DBA variant for New Zealand Colonials Wars. The author writes:
These rules are intended to refight the battles of the New Zealand Colonial Wars, 1840-1872. They do not cover skirmishes They are based on DBA, and influenced by the article by Ed White on DBA and the Seven Years War (WI65), and Clive Clapson’s article on the Maori Wars (WI63). As in Ed’s article there are two assumptions:
1. The first round of close combat also includes short range small-arms and artillery fire.
2. Troops potency depended on their ability to deliver close range volleys or cold steel.
Colonial Rules For Heroscape Terrain
If you are lucky enough to have acquired HeroScape terrain, you may be interested in these Colonial Rules For Heroscape terrain.