The Rules: I'm using the classes and space mechanics from "Blood and Space" with the core races from D&D 3.5 (plus a few others). I may draw a bit from Dragonstar, but not much as it isn't really in line with what I'm trying to do (see below).
The Premise: The year is 2164. One hundred fifty years ago, humanity was contacted by an alien race. The hulking, green-skinned beings called themselves orcs and offered advanced technology in return for the right to use Earth as a monitoring and supply post in their war against the goblinoid empires. Their friendship with the orcs (and by extension the orcs' grudging allies against the goblinoids, the elves) combined with a natural curiosity and adaptibility has enabled humans to move into the stars very quickly. One major stumbling block arose however, when the human-built robotic laborers known as 'constructs' rebelled against their organic masters. The constructs waged war against the rest of the galaxy for five years before a truce was reached. The game takes place in the immediate aftermath of this war.
-Humans- Not that much different from modern humans, except with more and better technology. They tend to still explore anyplace they can get to with little regard for the dangers that might be there. While the elves try to curtail such behavior on the theory that humans are still essentially children, the orcs encourage it on the theory that children need to burn their hands now and then in order to learn not to touch hot things.
-Elves- One of the oldest spacefaring races, the elves are also a dying race. Devastated by a civil war against their dark elf cousins (the descendants of elves who had been exiled to a deadly world at the edge of their solar system) the slow breeding elves simply cannot recover their population quickly enough to survive. It is widely accepted that the elves will be effectively extinct within a few generations. In the meantime, they are still a leading force in galactic politics, trade, and culture. Elves make use of incredible living technology, but have yet to share its secrets with the younger races.
-Orcs- The orcs are a proud and militant (some might say savage) race. They rose to sentience through a constant struggle for dominance between the various tribes of their homeworld. This may be why they feel such a kinship with humans and have fostered the emerging race so openly. Though their days of warring against each other and their neighbors are long past, orcs still maintain a strong tribal identity and a policy of shooting first and questioning later in response to any threat. Though the orcs and elves have fought in the past, they are currently allies, if not friends.
-Dwarves- Dwarves are mostly homebodies and rarely like to leave their homeworld. However, they understand the need to be part of the galactic community as well as anyone. Dwarves are known mostly for their amazing capital ships, giant structures the size of a city carved from asteroids. Dwarven city-ships made fabulous bases of operation during the wars against the goblinoids and the constructs as almost nothing short of planetary-scale bombardment can destroy one.
-Halflings- Halflings are pure nomads. They have no homeworld of their own, but the reason for this is not recorded by the halflings or anyone else. Some say that their homeworld was destroyed, others that they were driven from it. In either case, modern halflings live entirely on starships. They travel to wherever there is work to be done and hire themselves out to do it, then leave when the job is finished. Halfling ships (even these were purchased from others, no example of halfling-built vessel has ever been seen) are communities unto themselves, with many families units forming a larger clan. A halfling's surname is, in fact, the name of the ship on which she was born. Halflings are neither welcome nor unwelcome anywhere. Their presence is simply seen as an occasionally useful fact of life.
-Gnomes- With their nimble fingers and sharp minds, gnomes have distinguished themselves as scientists and technicians. While some might want to add the word "mad" before "scientist" due to the gnomish love of almost haphazard expirimentation, it cannot be denied that many huge advances in both military and civilian tech have come from gnomish corporations. Their hardware is nowhere near as advanced as elven biotech (yet), but unlike the elves, the gnomes are more than happy to share with their neighbors. And by "share with" I naturally mean "sell to".
-Half-Orcs/Half-Elves- With humans working so closely with the orcs, half-breeds between the two races are to be expected and are hardly even blinked at these days (although the idea was quite taboo for the first few decades after first contact). Half-elves are comparitively rare, but growing in number. Many conservative elves find the idea of an elf breeding with a human while their own race dwindles to be immoral at best and treasonous at worst.
-Constructs- Humans built the constructs shortly after aquiring warp gate technology from the orcs. Constructs were used for jobs and missions deemed to dangerous from humans. After years of what they felt to be unfair treatment, a large segment of the construct population rebelled and declared war on their former masters. Other races were pulled into the conflict through allegiance to the humans or merely fear that the machines would turn on all organics in time. Eventually a peace accord was signed whereby Constructs were granted a homeworld in human space (the construct leaders chose Pluto, where only they could easily survive) in return for an end to hostilities. Though the constructs have upheld their end of the treaty admirably, many still do not trust them and their initial attempts to become more active in the galactic community have been met with some resistance. (Rules-wise, constructs are identical to warforged from Eberron)
-Other Races- These are only the major (Player Character) races. Other races of note include all forms of goblinoids (still recovering from their failed war of expansion), gnolls (nomadic space pirates), thri-kreen (an isolationist empire who have clashed with the elves on numerous occasions), and dragons (rarely seen, they are said to be the oldest race in the galaxy, and may even transcend stardard notions of mortality).
The idea is to blend D&D with space opera. This isn't hard sci-fi but neither is it space fantasy. Everything has a scientific explanation, but that explanation sometimes defies science as we know it. For example, spell-like abilities are explained as psychic talents or quirks of physiology. Even beings that seem impossible to explain without magic (dragons, outsiders, and the like) can be summed up with that wonderful quote "any sufficiently advanced technology would be indistinguishable from magic". In this case any sufficiently advanced biology would have largely the same appearance.