Back in the 70s and 80s, Game Designers' Workshop (GDW) was the undisputed master of sci-fi rpgs. Of course, the flagship of their product lines was Traveller, which would later become MegaTraveller and the unfortunate Traveller:The New Era before veering off into several different directions with multiple publishers and game systems. However, GDW also published two other high-profile SF rpgs, the gritty post-apocalyptic Twilight: 2000 and the space exploration game, 2300 AD. These two games were set in the same timeline, three centuries apart, as the titles imply. The "Twilight" is a WWIII scenario in which a war between Russia and China expands outward into a global conflict. There's a limited nuclear exchange and a civil war breaks out in the US. In the aftermath, France, having chosen to withdraw from NATO and remain neutral, emerges relatively unscathed and becomes the new global superpower. Diminished versions of the USA (less an independent Texas and much of the southwest annexed by Mexico) and China (now divided into a strong Manchurian and weaker Cantonese state) rebuild in the subsequent centuries to join France as major spacefaring nations. In 2300 AD, these three nations lead the human race in the exploration and colonization of space.
Out of print for many years, 2300 AD has finally been revivified as an alternate campaign setting for Mongoose Traveller. The hardcover core campaign setting book is awesome. Weighting in at over 300 pages, it's light on artwork, mostly ship deckplans and planetary maps, but it's heavy on crunchy content. Of course, while the extensive stats on weapons, vehicles, starships and DNA modifications are useful and help to establish the feel of the setting, it is still the fluffier bits that make it interesting. 2300 AD presents a future that is, in many ways, a reflection of the past, in particular, the colonial period. Powerful nation-states compete with one another for habitable planets. Alliances are made and broken. Alien cultures are encountered. For those familiar with the original version, note that the first contacts with the Kaefers have been made, but little information on them (or indeed, any of the alien races) is revealed in the core book. Presumably a future supplement will be forthcoming.
Among the coolest things about the setting are the various alliances and rivalries that exist. For example, Canada has a long-standing cultural relationship with Great Britain and came to her aid in the aftermath of Twilight to help rebuild. However, Britain also has an economic and political alliance with the French Empire. The French Empire and Manchuria have a hostile relationship and have recently been in armed conflict. Canada, on the other hand, has a very good relationship with Manchuria and maintains several colonies within the Chinese Arm (i.e. Manchurian space). Admittedly, a lot of this high-level stuff probably has little effect on the day-to-day activities of the players, but it makes the setting feel real and immersive.