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.
one page
Age of Fantasy Regimental Rules
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’ Vietnam Rules
Andy Watkins offers a set of relatively simple Vietnam War rules.
One Page Kill Team
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
One Page Napoleonic Rules
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.