Rabbidgerbal’s Super Simple 1PG Sci-fi Skermish Rules is a set of free wargames rules meant as a 40K replacement.
Yes, that’s really how the author spelled “Skirmish”
Rules: Science Fiction
Space Fury Easy Starfighter Combat Rules
Space Fury are a set of quick and dirty space fighter combat rules.
One Page Fleets Starship Combat
One Page Fleet is Mike Wikan’s set of free wargames rules for starship combat.
Star Wars Shootout
Star Wars Shootout is a set of free wargames rules for skirmish combat in the Star Wars universe.
Iron Stars demo rules
Majestic 12 Games has these demo rules for their Iron Stars miniatures wargame. I’ll let them explain:
Iron Stars is a game of space combat in an age of steam and steel. Set in a fictional universe based loosely on the writings of H. G. Wells and other fantasists, it provides a back story and ship designs from an alternate history in which the Martian Invasion actually happened, the ether is all?pervasive, and Cavorite is a reality. But, as with Starmada and other Majestic Twelve Games products, the focus is on players’ imaginations; a clear and concise ship construction system is provided so you can pit your own space dreadnoughts against the likes of the Royal Navy Ether Squadron or the German Aethermarine.