Shako and Drum is a set of free wargames rules for the Napoleonic wars. It’s a one-page set of rules, for those who like that sort of thing.
Age of Fantasy: Regiments is a set of free wargames rules for simple fantasy Regimental actions. The rules fit on one page, and there are “army books” for various races.
Andy Watkins offers a set of relatively simple Vietnam War rules.
One Page Kill Team is a set of free science fiction rules for small skirmishes. The authors write:
One Page Kill Team is a single-page ruleset designed to be fast to learn and simple to play. It is a great way to play with just a handful of miniatures and experience heroic skirmishes in a short time.
– Updated January 2015
Land of the Lead has a set of one-page Napoleonics Rules. They’re designed for play with a “reinforced division” per side. Scroll down to the bottom of the page to find them.