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 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 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.
Tired of Warhammer 40K? Try Slammer a set of free wargames rules by Chris and Alex Nicole. The rules set is not tied to any particular set of science fiction miniatures, and there are army lists for a wide variety of troops — even Zulus! ??? (For those hordes of primitives versus superior technology games).
Corporal John is a set of free wargames rules for the Age of Marlborough. The author writes that:
The set can be used, not only for Marlborough’s wars, but for those of Charles XII, Peter the Great, Louis XIV and even Gustavus Adolphus.
The rules are fast-play but still retain the detail to give the correct flavour of the period. They will handle games of thirty or forty units per side or much smaller games with a handful of units.