Compared to the Free Fantasy OSR Rulesets, the Science Fiction Collection of Free OSR Rulesets is very much smaller. Perhaps my Google-Fu has failed.
If you know of Free Science Fiction OSR Rulesets that aren't yet in this collection, leave a comment below and I will be glad to update this page.
Note: I'll be working on Free OSR Adventures next.
Stars Without Number - Revised Edition - "Stars Without Number: Revised Edition is an old-school-inspired game of sci-fi adventure, one built from the ground up to encourage sandbox play and simplify a GM's job in providing it. Familiar mechanics are employed to forge new worlds and explore new possibilities, guidelines built on long experience used to help a group venture in wholly new directions."
Mutant Future: Revised Edition (no-art version) - "Mutant Future is a post-apocalyptic science fantasy RPG, in the same flavor of similar genre games from the late 70s and early 80s. The game is almost entirely open game content, and a free trademark license allows anyone to publish materials compatible with Mutant Future."
X-plorers - "Here is a simple RPG of interplanetary adventure, rules-lite, fast-paced, and inspired by a passion for science fiction. In X-plorers, you're part of a group of galactic troubleshooters on the look out for the next job-whether it be salvage, search, or rescue on an alien world."
D6 Space - "This book provides the fundamentals necessary to play in nearly any type of science fiction setting using West End Games' famous D6 System roleplaying game rules." Aliens, Androids, Space Opera and Space Ships
Septimus (d6 system) - "The game of Septimus takes place in a galactic civilization that has run its course and now Is going through an extended death spasm. This civilization Is the vaunted Seventh Empire, the greatest single political entity ever crafted by humanity."
Hulks and Horrors - Basic Black Edition - "We are the Surveyors. We explore the uncharted sectors of space and hunt these ruins for the treasures they contain and the riches they might bring to what sentient life remains. We are not brave, merely foolhardy and desperate. Those who survive become rich beyond dreams. Very few survive. Hulks & Horrors is a science-fiction roleplaying game of galactic exploration and adventure for 2 or more players ages 13 and up."
Cepheus Engine System Reference Document - "The Cepheus Engine System Reference Document comprises the core rules for a Classic Era Science Fiction 2D6-Based Open Gaming System. Inspired by the original three books of the oldest 2D6-based science fiction roleplaying game, the Cepheus Engine system incorporates elements of Open Game Content from previous releases of that game of intergalactic drifters and wanderers to create a basic system that will allow players and Referees alike to create their own science fiction universes or play in the ones they are already familiar with."
Cascade Failure - "Cascade Failure is a post-apocalypse future RPG. Set in the aftermath of a devastating war that collapsed the interstellar economy, Cascade Failure is a survival RPG that casts the characters out into a dangerous and hostile universe. The game is built on a rugged OGL chassis that is melded with a variety of new subsystems to round out your character. You can jump into unique character classes like the Kinetic that can move objects with their mind or into a unique alien species like the insectoid Sahael with a hardened exoskeleton and powerful mandibles for biting attacks."
TERMINAL SPACE - "In the beginning (these rules) were to comprise a separate game partially based on simplified Moldvay / Cook edition of the game and partially on my own solutions. After some thought, I came to the conclusion that my works would be better suited as supplements for the Original Edition. The aim of this publication is to add a new rule set to help transfer the game to an entire new dimension which is space exploration. Thus these additional materials were created with the thought of settings more heavily based on Sci-Fi and Science Fantasy genres than most of the materials created for 0e so far. Even though this supplement is not complete I hope that somebody will find it useful."
The Frontier Starter Edition - Inspired by action roleplaying games, such as Borderlands and Destiny, The Frontier is a sci-fi game of hi-octane action that focuses on the simpler pleasures of life. Explore the alien ruins of a faraway world, fight a ton of nasties and baddies, and collect a boatload of loot as your reward!
Apes Victorious - Apes Victorious is a roleplaying game in which you take the role of an astronaut from the 1970’s who finds himself marooned on a future Earth ruled by intelligent apes. Players may also take the role of an ape, a degenerated human, or a psi-active underdweller. Fight to survive in this post-apocalyptic future ruled by four species of apes. Or play apes who hunt humans for sport. For a different kind of campaign, play highly intelligent but insane underdwellers who have advanced technology and powerful psi powers.
Mothership: Player's Survival Guide - Mothership is a sci-fi horror roleplaying game where you and your crew try to survive in the most inhospitable environment in the universe: outer space! You'll excavate dangerous derelict spacecraft, explore strange unknown worlds, exterminate hostile alien life, and examine the horrors that encroach upon your every move.
ICAR - Icar is a space opera, more light-hearted than hard science fiction. It is set in a distant future where a benevolent Imperium protects the human race, which is battling a race of killer robots it created!