Boonta Eve Classic is a set of free wargmes rules for pod racing in the Star Wars Universe. The rules are designed for use with those old Galoob Micromachines, which can still be had for a reasonable price on Ebay:
science fiction
Anaxis Space Combat Game
Anaxis is a free space wargame, played on a hex grid with ship counters.
Away Team Astro Miniatures Skirmish Game
Away Team is a set of free wargames rules for a Star Trek-style landing party expeditions.
There’s also a set of 15mm not-Star Trek miniatures.
Firefight Mass Combat
Firefight Mass Combat is a set of free wargames rules originally written for use with Alternative Armies’ 15mm science fiction ranges.
Alien Invasion Rules
Matakishi has a set of free skirmish rules called “Alien Invasion”. The author writes:
This is a skirmish level game designed to be played by four to six players in about an hour and a half. Each player will control a competing faction with personal victory conditions to achieve based on the scenario chosen. Not all victory conditions are mutually exclusive so alliances and diplomacy have their place.
The game is ostensibly set in a fictional present day but this is not a requirement. Many of the factions are drawn from popular culture and new factions can be added very easily simply by quantifying a small number of variables.
The game requires minimal measuring, no charts and a lot of six sided dice. The rules will be memorised by a player by their second action and everyone can then concentrate on having fun.