Arthur Chang has written a set of add-on rules for playing massed battles with the Wizards of the Coast rules for Star Wars miniatures.
rules
Ancients By Ral
The Jackson Gamers offer Ancients By Ral, an ancients version of the venerable Rules By Ral system.
Angels 1-5 World War II Air Combat Game
Angels 1-5 is a game of air combat over Britain in 1940
Star Breach Sci Fi Rules
Star Breach is a set of science fiction rules that are miniatures agnostic. The free 1st edition core rules are available at the link. There’s also a deluxe version you can buy.
Planet 28 Sci Fi Skirmish Rules
Planet 28 is a set of free wargames rules for “quick narrative games between warbands of warriors, mutants, and outcasts.”