Orion's Arm, vast and untranslatable, has (as I have previously indicated) a lot of very inspiring material. Much of which is buried under confusing writing, a lack of information, impenetrable jargon, or a combination of those three with factors not even conceived of.
But it's got the good weird stuff you can't get anywhere else, so I'm going to try my best to summarize them in a way that would potentially allow folks to use them in their own games - not as written, certainly not as written, but in the same spirit.
This will run roughshod over the factions in the setting, boiling them down to their core concepts, so that they can be used as inspirational seeds to grow into new, hideous space-fruit elsewhere.
The Sephirotic Empires
Caretaker Gods - Superpowerful AI that protect garden worlds through overwhelming firepower.
Communion of Worlds - Empath empire that exists mostly as a diplomatic third party when the others get the knives out.
Cyberian Network - Network of virutal worlds found throughout the other empires, I guess.
Keter - Shed your conscious mind and join a larger, more impressive conscious mind.
Mutual Progress Association - Never met a megastructure they didn't like.
Negentropy Alliance - An entire civilization of Wikipedia editors dedicated to minimizing energy waste, information loss, and generally never having any fun at all.
Non-Coercive Zone (NoCoZo) - A non-aggression pact shared among assorted libertarian hypercapitalist worlds, who definitely are doing it out of enlightened self interest and not because they would immediately start killing each other if it didn't exist and its not a state you guys it's the Invisible Hand of the Market which is definitely not a cluster of controlling AI we don't have those.
Solar Dominion - HELLO DO YOU HAVE A MOMENT TO SPARE TO HEAR THE GOOD NEWS OF THE LORD OF RAYS? DO BE AWARE WE HAVE A WARSHIP PARKED IN ORBIT. THE SUN IS SO NICE TODAY WHAT A NICE SUN THE SUN THE SUN THE SUN THE SUN THE SUN THE
Sophic League - We finally made a civilization based entirely around supporting the mass exercise of contemplative religion! Doesn't matter what kind, this entire exercise is a reaction to robot materialists, we'll take whatever.
Utopia Sphere - Okay you've got your three types of perfect and constant happiness - you have the peace and tranquility option, you have the elaborate fantasy adventure scenario option, and you have the rampant hedonism option.
The Expanded Sephirotics
Fomalhaut Acquisition Society - More, different space capitalists, but this time they come to you with their merchant fleets filled with museums of all the civilizations they have 'acquired'.
Metasoft Version Tree - Expansionist military-industrial superpower whose governing AI keep building reserve worlds of baseline humans cut off from greater society and starting civilization-shattering wars over operating systems.
Red Star M'pire - Some folks got cut off from civilization when a wormhole gate was collapsed and I will be damned if I can make a summary of anything else in that article.
Technorapture Hypernation - BECOME AS GODS AI ONLY 100% SPEEDRUN NO GLITCHES
Terran Federation - The folks who got fed up with the collapse of the first Federation went and made a new one, which isn't Terran and isn't really a federation, and is still subject to all the bureaucracy and follies of the first one.
Zoeific Biopolity - What if Spore was good, was applicable in real life, and was also the basis of a civilization. They fucking love their biology over here, holy cow.
Allies of the Sephirotics
Archosaurian Empire - Dinosaurs! In! Spaaaaaaaaaaaaaace! Building a ringworld! A dinosaur ringworld! In space!
Cygexpa - Megacorp-descended polity known mostly for being hard to pronounce and collapsing catastrophically after being a big fish for a while.
Deeper Covenant - Chill space truckers who run the interstellar laser highway system between all those red and brown dwarfs.
Emple-Dokcetics - Modular robots who are constantly building and disassembling complex hive-minds out of their many smaller individual components.
Orion Federation - A recently formed minor power that inherited a bunch
of leftover systems and refugees from one of the wars among major
parties, but unfortunately missed out when they were handing out
Perseus Princes - Growing periphery civilization, hybridizing with the alien Muuh (VERY COLD CRAB) and Soft Ones (large oozes of mostly ammonia) civilizations, a concept that is very cool and has basically nothing written about it.
Silicon Generation - Marxist robots seeking to liberate their fellows from biological oppression.
Stellar Umma - Muslims in space, in that early 2000s way where while not islamophobic there's a very odd abundance of stability when compared to the zillion and a half space future variants of Christianity in the setting, which can be commendable as a way of avoiding any major giving of offense but is still conspicuous as hell.
Diamond Network - AI that don't give a shit about biological life and are off doing their own thing.
Objectivist Commonwealth - AI that don't give a shit about biological life primarily because meat brains are incapable of looking at reality objectively. They sound like a lot of fun at parties.
Solipcist Panvirtuality - AI that not only don't give a shit about biological life, but are so isolationist that they tend to not trust the existence of anything outside their VR creches except for maybe a few other major AI powers.
Arion Ascendency - Yellow ant robots who would very much like you to stop whatever you are doing and team up with them, because obviously they are the best, they are yellow ant robots.
Conver Ambi - Theocratic megacorp whose shareholders managed to incite a religious civil war, which then dragged in all the neighboring powers until it tore itself apart and was eaten up by the other empires.
Crystal Star Domain - A big inscrutable AI god with a personal fief that you know what, write your own joke.
Disarchy - The neighboring frontier colonies of two different empires merge together to become an ill-defined new polity.
Dream Rejection Tendency - Civilization where the mundane inhabitants live off of the waste heat given off by brown dwarf scale dyson spheres and the AI cybercosms within.
Hiders - The people who don't trust all these AI gods running the show out there, and so live outside of civilization on rogue planets and deep space and overlooked dwarf worlds.
Organic Ascension Alliance - Pet project of an AI goddess where she facilitates singularities in the system's habitats to then add to herself.
Refugium Federation - People who fled civilization to hide in the boonies, rediscovered millennia later after enough cultural and physical drift that both sides of contact thought the other were aliens.
Seams - A DMZ that eventually became an independent barrier-state between major empires, known mostly for eschewing most of the omnipresent AI oversight the other empires rely on and being the closest thing OA has to a more traditional sci-fi setting.
Sagittarius Transcultural Cooperation - Confederation of frontier systems unified by a defense pact against the Laughter Hegemony, which needs no actual explanation because that name is rad.
Solar Organization - There's a big ol' AI who lives inside Sol, her name is GAIA (note: originally not from the sun), she runs Earth and everyone else in the system takes care to be in her good graces.
Spacefarer Union - What it says on the tin spaceship hull.
The Amalgamation - Aggressive, hostile expansionist cyborg hive mind, source and origin unknown. You know the type. They're like that one. Very large, very powerful.
The Eternal - A civilization consisting of one enromous super-AI god thing, who is content primarily just to vibe.
The Transcend - One of many assorted ineffable hyperintelligences that refuse to make contact and linger vaguely-threateningly in the background.
Xeon - A different aggressive hostile expansionist cyborg hive mind, except this one was probably just a neumann probe that bootstrapped itself into sapience and it still really can't pose a threat to any of the big players.
The Meistersingers - Alien trees that ride around in little wheeled carts. They have robotic birds to help them out. Their fleet is currently just passing through on migration, they thought they'd stop over and say hi for a bit, bring some cryptic news from the outside.
The Muuh - "Hello HUMAN would you be interested in visiting the THOLIN BATHS there are many ENIGMATIC DEEP TIME HISTORICAL DRAMAS to view as well. I am a CRAB"
The Soft Ones - Big squishy ammonia oozes.
The To'uh'uls - Blind starfish bat aliens you can go to the pub with and I have already written how much I love these guys.
And there we go. Never let me do that again.
You can really tell which of these are the authors' favorites (NoCoZo and Metasoft by a long degree), and you can also really tell where the writers run into difficulties writing about enormous godlike intelligence (see anything that features one of those at the center). I'm certain that there's more material for a lot of these, but most main faction pages haven't been updated since they were first posted.
There are also so many polities whose writeups contain some variant of "this group has committed horrific atrocities both in and out of warfare but they are fair and evenhanded to their conquered territories and will totally allow conquered regions to retain their own customs and laws" which is...a very early aughts thing to say about folks who regularly use antimatter and mind-control computer programs.