In 1974, TSR published a set of rules for playing out battles set in Edgar Rice Burrough’s Barsoon.
One Page Kill Team is a set of free science fiction rules for small skirmishes. The authors write:
One Page Kill Team is a single-page ruleset designed to be fast to learn and simple to play. It is a great way to play with just a handful of miniatures and experience heroic skirmishes in a short time.
– Updated January 2015
Wandering Star is a set of free wargames rules for science fiction games with a Star Wars or Firefly vibe. From the author:
The design philosophy behind the rules is grounded in the idea that we want our sci fi games to feel like episodes of Star Wars or Firefly. This means that I deliberately tried to keep away from mechanics that model modern warfare. For example, weapon ranges are rather short (even though I know about the discussions surrounding this topic and would chose a different approach for historical games). Wandering Star is not supposed to force you to adopt modern small unit tactics, but to force you to behave like heroic rebel commandos in Star Wars!
Icarus Miniatures offers a set of preview rules for their Icarus Project science fiction rules.
There are also some starter paper miniatures and unit cards for the downloading.
Senator Arjun’s rules are for games in the Star Wars Universe. Each player controls a squad or more of figures. There also are rules for jedi, droids and various villians