The Jackson Gamers offer Ancients By Ral, an ancients version of the venerable Rules By Ral system.
rules
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.”
Warpath Rules
Mantic offers its Warpath science fiction rules free for the downloading.