There is an Aliens book available for it, which gives all the aliens mentioned in the Classic Setting book Species sheets, making them playable, and going into great detail on the Uramkup, the Guaru, and the Kertu-Drua aliens.
*Zero Stage* - This game is a new setting entirely, though it CAN connect to a more typical Cluster. It is a ten star system mini-empire called Jeshen Space. It is run by the Jeshen, and Humans came in as refugees. This game has a lot more politics and cultural clashing than most, exacerbated by default PC association being a news organization, and the PCs being news teams. You can play as Jeshen or Human out of the box, which introduces a possible awkward factor, as Jeshen are very humanoid and are all hermaphrodites. I don't know your gaming group, but mine had no problem. There are all kinds of possible trouble the PCVs can get into - possible cabals and conspiracies, an alien threat, the issue of sapient slavery, attempts to drive a wedge between Humans and Jeshen, as well as the old murder and theft.
*Dark Orbital* - This game is set within in the Classic Setting, a single system (Cry In The Dark) is described, but it really concentrates on a single Orbital/Space Station. The eponymous Dark Orbital is a converted STL generation ship from Earth, and is about two thousand years old, and the PCs are the poorest of the poor - the forgotten uplifts and dropouts living without any access to banks or credits, trading ceramic tokens back and forth, and repairing and reusing old technology disposed of by the rich. Everything these people own is either made by them or repurposed by them, yet their culture is rich and interesting, and their ability to cobble something cyberpunkish together out of junk is ingenious. You can play Human or uplifted Hyena, Dog, Cat, Raccoon, or Hare PCs.
*The Necklace* - This game is set in a strange solar system, with a regular sun and a neutron star which has a gigantic torus of air around it. Inside is a billion mile long manufactured river, maintained by alien gravitics, and studded with asteroids and teeming with life. All around it is freefall life - spherical seas and puffball jungles. The setting is wild and strange, and may be intimidating to some. There are several different varieties of humans to play - mostly cultural differences, though Altisherpas have been genetically tailored to live in freefall - amd one alien species, the Pucks. Pucks are winges, and can fly in freefall, and are only sapient when combined with a brain parasite that sits on their head, tunneling through their sculls. The Necklace comes with an Association creation tool, so you can define what your PCs do in the setting. We had a great time playing actor/thieves, journeying around the River on a showboat, putting on plays, romancing fans, and stealing stuff.
*Cold Space* - This game is an old one, system gutted out and replaced by the new one, and is not in the least related to the old StarCluster game. it is set in an alternate universe, starting in the late 1950s, just after WWII. Really old, primitive atomic rockets, but with the Solothurn Drive - a combination contragravity and strange "FTL" drive, which did not actually make ships go faster than light, but compressed space around the ships. The Cold War is played out in space between the late 1940s and the fall of the Soviet Union in 1989. There is a burst of colonization of nearby star systems - the Oikumene, as it's called - and trade begins. The main colonizers are the US, The Soviets, The Commonwealth, and the UN. Aside from the Solothurn drive, everything is real tech, either actually existing or partially developed but abandoned. This may be the easiest setting to grasp. It presents a lot of colony worlds in fair detail, but it does not have an association mechanic, though it does have recommended campaigns and characters.
*FTL Now* - This is a literal sequel to Cold Space, set from immediately after Cold Space ends, 1990, to now. The centerpiece of this game is the slamming of a mars-terraforming comet into New York Harbor on September 11, 2001. Thus the war on terrorism moves out into space,. Earth is hurt horribly, with billions dying. The colonies are left on their own, trying to make do without or by trading with their neighbors. Some colonists, called angels, return to earth to help in the devastated areas. The Oikumene is bigger, the older colonies are best off, and they all do their best while the US Rocket Corps attempts to root out the terrorists. Everything I said for Cold Space applies to this game as well, plus it is set in a time period your players will be very familiar with.
*Sweet Chariot* - This game is set within the Classic setting, with a detailed star system - the Gloria system - but it's really about one odd world. Chariot is a world with a thick argon atmosphere, with just enough oxygen to live. Argon is poisonous at a certain pressure, so the Humans and animals have to live in the mountains. it is also hideously hot, and rich in radioactives, so much so that natural reactors mediated by groundwater occur. Why would anyone live there? Well, hijackers attempted to take over their ship in between Earth and the Cluster, and the landers, a lot of tech, and much of the biological cargo were destroyed. They chose to land on Chariot because they could cobble together awkward simple landers that would have a better chance of landing. The landers got down, but in attempting to bring the generation ship down, it crashed. The landers were scattered over Chariot in various isilated high terrain. They evolved into separated countries with totally different cultures, and they fly between them in steam powered airships. So this is a wierd kind of steampunkish place, in contact with the vastly superior cultures on other planets in the system, but going their own ways. If your players enjoy thak vibe, this game is for them. The different nations are very fully described, so there is a lot of setting info. No association rules - though those are available in the Toolboxes, but this is one of my favorite settings to play in!
*Out Of The Ruins* - This is a very odd setting, but I am having a lot of fun with it! It is two star systems and a newly transluminal empire, with all the species being successor species to the Humans. The Humans are gone, and their successors are searching for them. They have taken the book The Silmarillion by Tolkien as their holy book, and interpret everything in the terms of this book. They speak Elvish to each other as their holy tongue. There are many playable species - New Humans, who live in trees and have prehensile tails, seven kinds of Dogs, Giant ravens called Corvoids, Kangaroos, Neo-dolphins, a cat-centaur thing called a Bintaur, and alien Groar, Alish, and Karkris. There is a war going on between the Neo-Dophin-Alish alliance and the Bintaur-Karkris alliance. The ships work like the ones in Cold Space and FTL Now. The Church are scavenging what ruins can be found on a world once settled by the Eldar (AKA Elves) but now undergoing glaciation - the current culture is built on technology gleaned from these ruins. If your players like the wierd and enjoy Tolkien, this might be dun for them. I recommend getting the Helkaraxe supplement, which details a rogue gas giant and its moons out between the stars.
*Sabre & World* - This game is a straight out Sword and Planet game, like the John Carter Barsoom series by Burroughs. you can play either a single Earth Human transported to this new world with the rest of the PCs being of this world, a group of Earth Humans transported there together, or a group of people from this world. You can play Earth Humans, Humans from this other world, or a Centauroid people. The world is generated, though there is a play area pregenerated for you if you prefer. You can expand it or generate other areas later. The mechanics are very different. Emotional ties to other characters give you big benefits, and weapons do only one point of damage per hit. Damage is assigned directly to your attributes, via status boxes. These changes completely change the feel of combat. The world you go to is a world which used to have high technology, but now very few people undertand it, and the high tech is mostly rare and old. This all embraces the Sword and Planet style tropes, and tends to produce that kind of game. If you like Sword and Planet stories, or are intrigued by the idea, this is what you want.