John Bear Ross, digital sculptor extraordinaire, offers HELLION, a set of universal miniatures rules.
Here are a set of Very Simple Generic Miniatures Rules.
Core Game Development Studios offers a set of free wargames rules called Timeline. From the author: Timeline Skirmish is the skirmish game covering all historical ages and into the far future. This game is meant to be fast and furious.
Bunkermeister offers Quick and Simple Wargames Rules For Everything.
David Medlock offers The General Purpose Wargame, a set of semi-skirmish level wargames rules for simulating battles from the Stone Age to the Victorian.