Bob Cordery’s Bundok and Bayonet free colonial wargames rules are available here. They are a clean, well-written set that use a card activation system. Two things I really like about these rules: 1) that you check morale before moving 2) a bucketsful of dice mechanism for combat.
Rules: Colonial
Front Rank Fire Zulu Skirmish Rules
Front Rank … Fire is a set of free wargames rules for the Zulu War of 1879. The game is for skirmish actions in 15mm to 25mm figures, and you’ll need a ratio of about 3-1 Zulus over Brits to make it a game.
Warhammer Colonials!
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.
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.