Death Before Distemper is a set of free wargames rules for science fiction platoon level games.
Miniatures Rules
Bombs Away Carrier Game
Bombs Away is a simple, three page print-and-play game about carriers in the Pacific.
Spectre Modern Operations Rules
Spectre Miniatures offers a set of free wargames rules for modern operations called Spectre. Click on the “rules” tab at the top of the page.
There’s also a line of 28mm miniatures that you can buy, if you don’t already have some. I’ve got moderns in 15mm already.
The Battle of Blackburn’s Ford
The Battle of Blackburn’s Ford is an introduction to the Combat Command American Civil War Rules. The pdf also includes counters if you don’t have figures.
Red Army White Guards Russian Civil War Rules
Red Army White Guards is a set of free wargames rules for playing battles set in the Russian Civil War. Its played on an offset grid and is apparently designed for 15mm figures; the rules call for three figures on a 30×30 mm base.