I wish I could remember all the science fiction wargames that I've come across on the internet, but it all gets jumbled together, and I've never made a list anywhere. Since Raggle is talking about alternative lore to Warhammer 40,000, here are some, broken down by fictional setting (I'm sure there are many I'm missing):
Warhammer 40,000 / Warhammer 40,000: Kill Team / Battlefleet Gothic / Space Hulk / Necromunda / Gorkamorka / Epic 40,000 / Adeptus Titanicus / Aeronautica Imperialis / Warhammer: The Horus Heresy
by Games Workshop
The carrion Emperor sits on his throne. He is master of a million worlds. In the grim darkness of the far future, there is only war. Most of these games are in the 28mm scale. Warhammer 40,000 is a full wargame in 28mm. Kill Team, Necromunda and Gorkamorka are skirmish games, using the same 28mm miniatures. Epic 40,000 is on the 6mm (huge warfare) scale. Battlefleet Gothic covers capital ship space combat. Adeptus Titanicus and Aeronautica Imperialis cover Titan-sized mechs and aerial warfare respectively. The Horus Heresy is a 28mm prequel set 10,000 years before Warhammer 40,000.
Factions: Imperium of Man, Eldar Craftworlds, Eldar Exodites, Dark Eldar, Leagues of Votann, Ork Empires, Necron Dynasties, Tau Empire, Tyranid Hive-Mind, Realm of Chaos
BattleTech / BattleTech: Alpha Strike
by FASA Corporation, currently Catalyst Game Labs
1000 years after the first space flights, humanity's diaspora has developed the Inner Sphere, a region of space containing hundreds of systems. The classic giant mech wargame that spawned the long-running Mechwarrior series of mech sim games on PC. You customise different mechs, adding different guns for different situations, and act as a mercinary for the human political powers that have spread across space. It is quite hard science fiction with only human political entities. The different far-future states have adopted elements of feudal dynastic monarchy, meaning that mechs are often inherited as a knight might inhert arms and armour.
Factions: Federated Suns, Draconis Combine, Lyran Alliance, Capellan Confederation, Free Worlds League
Star Fleet Battles
by Amarillo Design Bureau
Once, it was one of the most widely played wargames I hear, and influencial. Star Fleet Battles was a Star Trek wargame created to simulate fleet engagements, but effectively got frozen at the point of Original Series era of Star Trek lore. It has since diverged into essentially it's own Star Trek universe based on the Original Series, with some empires that were created only for this setting, such as the Hydran Kingdom. I think there is a lot of really in-depth customisation of miniature loadouts, down to shield syetems and torpedoes. Modern companies have acquired the licence in recent years, so there was a FASA wargame extension of their Star Trek RPG, and there is currently an X-Wing style game called Star Trek: Attack Wing.
Factions: United Federation of Planets, Klingon Empire, Romulan Star Empire, Gorn Hegemony, Hydran Kingdom
Star Wars: Legion / Star Wars: X-Wing / Star Wars: Armada
by Fantasy Flight Games
A galaxy-spanning civilization plunged into war by the machinations of Dark Lords. Fantasy Flight Games probably emerged as the most serious competitor to Games Workshop. They have a similar 28mm scale Star Wars tabletop wargame, Star Wars: Legion. They have an Aeronautica Imperialis type game, Star Wars: X-Wing. They have a capital-ship combat game, Star Wars: Armada. Additionally another company, Atomic Mass games, has recently published a skirmish scale game that can be played with only 4-5 miniatures, Star Wars: Shatterpoint. It's tempting, but I don't want to burn myself out on miniatures, Warhammer 40,000 is my area of focus.
Factions: Galactic Empire, Rebel Alliance, Confederacy of Independent Systems, Galactic Republic
Infinity
by Corvus Belli
An anime-inspired but relatively realistic aesthetic. A post-cyberpunk space opera setting with fairly hard science factions. Infinity is probably the next most popular sci-fi skirmish scale game outside of Warhammer 40,000 and Star Wars. The major factions are based on pastiches of real-world cultures, as is often the case in science fiction wargaming factions. The PanOceania faction represent a noblebright democratic ideal. The Yu Jing more Asia-Pacific inspired. Ariadna is a partially French-inspired chivalric faction, incorporating elements of European noble ideals.
Factions: PanOceania, Yu Jing, Ariadna, Haqqislam, Nomads, Combined Army, ALEPH, Tohaa
Dropzone Commander / Dropfleet Commander
By Firestorm Games
Set in 2670 AD. An epic scale wargame about the battle of humanity vs an alien force invading the colonies of humanity as well as a battlefleet scale space wargame in the cold void. The minis look really nice on a table next to dense urban buildings, as it tends to be played. The space vessels also look very cool. It seems to be fairly hard science fiction, with human cities looking relatively like modern cities on Earth, similar to something like BattleTech.
Factions: United Colonies of Mankind, The Scourge, Post-Human Republic, Shaltari, The Resistance
Deadzone
by Mantic Games
Basically someone created a Warhammer 40,000 clone, I imagine probably marketed at a). people who want to pay less for their miniatures, b). people who like old Warhammer 40,000 design aesthetics, c). people pissed off at Games Workshop for their business practices or management of lore or changes in rules. I don't know if any former GW people were involved, but wouldn't be suprised. It's like Pathfinder to D&D. Marauders look exactly like Orks from 1st edition and 2nd edition, complete with silly grins, but are meant to be "mutants". Likewise the Veer-Myn are Skaven in space.
Factions: Enforcers (Space Marines), Forge Father (Squats), Veer-Myn (Space Skaven), Asterians (Eldar), Marauders (Orks)
Blkout
by Blkout
Set on a world that is humanity's only colony in space; Abol. A new one that I don't know much about yet, but seems to again have cool enough miniatures. It's operating in skirmish territory, in the same territory as Warhammer 40,000: Kill Team from what I can tell. The theory is that people can get into skirmish games with fewer miniatures. Again though the lore is pretty generic so far, as I can see, as you have the usual pastiches of some Earth cultures extended into the future. For example the Manticor are formed from "Eastern Bloc" nations, according to their description. So expect stereotypes born of the now 30-years dead Soviet Union, like "heavy armour".
Factions: Manticor, Harlow Kinetic Solutions, United Nations
Steel Rift
by Death Ray Designs
Set in the near future, in the wake of crippling solar flares. Looks like these guys are going for the Battletech market. Battletech itself has undergone a revival in the last few years thanks to 3D printers I hear, and is beginning to return to being a healthy widely-played game. Steel Rift it seems will present an alternative for people wanting to play a giant mech game, giving three options: Battletech, Steel Rift and Adeptus Titanicus. Again I'm not sure about the low-alien, low-space, corporation-heavy, setting (I find these things can be bland). It can be good if handled well, or outlined well then left to the players to write their own fiction.
Facitons: Freelance, Corporate, Authority
They all sound interesting. You know me, I love space. I find more science fiction settings always welcome. Some of the lore sounds promising. I think the main risk with anything nowadays is potential for wokeness. Independent efforts, perhaps simply giving people rules, letting them 3D print models bespoke, could potentially avoid corporate propaganda. But, science fiction is getting more woke overall, accepting the mass immigration narrative, where citizens are fungible units.
To be stocked in a shop, or to get investment money, likely means accepting the agenda of globalist investors like Blackrock or Vanguard, and I'm not sure how easy it is to start without capital. You might find that a modern setting is more prone to anti-white neoliberal propaganda, than a legacy franchise like Warhammer 40,000, which has 40 years of inertia and momentum to overcome before activists can ruin it.