Double Tap is a set of free wargames rules for games involving special operations forces in the near future. The rules are just one page, but look as though they are anything but simplistic, as they use a reaction system.
Andy Watkins offers a set of free wargames rules called Brigade Commander, for games set in Cold War Germany. Given the equipment used, I suppose it also would be useful for gaming some other scenarios in which the combatants have old NATO and Warsaw Pace equipment.
Italian wargames magazine Dadi&Piombo (Dice and Lead) is offering a free set of modern skirmish wargames rules called Breaking News
In the Noonday Sun is a set of rules for small unit actions, derived from another set called Slammer. The authors of Noonday write:
In the Noonday Sun is a set of wargame rules for skirmish games using miniatures. I intended from the start that Noonday Sun would be flexible and adaptable to different scenarios. I have intentionally kept it ‘loose’, particularly in areas like troop motivation and Hollywood heroics. Noonday Sun is meant to be fun! We use miniatures on the tabletop to represent men and women who might well be scared or heroic, angry or bored, professional or just hungry, above all they are unpredictable. When you play Noonday Sun, occasionally try to see things through your troops’ eyes. They are more than just disposable weapons delivery systems.
The Simple Toy Soldier Game is a set of free wargames rules from the Death Zap blog. It is designed for quick skirmishes with 4 – 8 figures per side and modern weaponry.