“a versatile, yet simple turn-based strategy game that is fully customizable. The game is designed to make use of Mega Bloks Halo sets, but you can use any kind of equally-sized figurines including cheap army men, and is meant to be played on any kind of table or flat surface. Similar to games like WarHammer 40k or even chess, Frontline Skirmish contains very loose, simple rules and maximum customization.”
The game is on the pay-what-you-want model.
The Osprey Rules A Fistful of Kung Fu 
Andy Watkins offers a set of skirmish rules for the Wars of the Roses. Watkins writes:
25mm Skirmish wargaming with Wars of the Roses figures was one of the first Skirmish games I ever played about 20 years ago. To be honest I haven’t played much for quite a while. The rules I used were a bit cumbersome, I’ve upgraded now to a home grown set that owe more to my Pirate rules than anything else. Though there is still a touch of influence from Retinue, the best of the commercial Wars Of The Roses rules.
Andy Watkins has these free simple skirmish rules for fighting miniature battles set in the French and Indian War. These rules belong to the “less is more” category.
Empress Miniatures offers a set of free wargames rules called Damned Wobbly Gentlemen, designed for recreating battles of the Zulu War.