Edwardian Splendor is a set of free wargames rules for playing games with 54mm figures.
Ross MacFarlane offers a set of set of free wargames rules for the late 19th/ early 20th centuries called Sending MacDuff to the Front.
The Dice Flew Furiously is Ross MacFarlane’s set of rules for modernistic warfare. From his notes:
Notes: This is a simple set of rules for playing wargames in the Age of the Machine Gun. They are meant to provide an enjoyable game and to be evocative of stories told rather than an accurate simulation or recreation of experience or fact. More specifically, they aim to work in OSW fashion, not worrying about consistent time and ground scales and operating on multiple levels at once. Figures and vehicles are treated more or less as individuals so a player can get that emotional feel of being there in the midst of things while putting dozens if not hundreds of toy soldiers on the table and getting the feel of commanding a “battalion” or even a “brigade”, yet still be able to finish a game in 2 to 4 hours. The rules are intended for use with individual figures of 20mm size or larger but it should be able to be used with multi-figure bases either counting each base as a figure or by marking hits.
Mexican Revolution is a set of free wargames rules for 25mm – 28mm figures. In them, a unit is represented by 10 figures.
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.