I've recently begun GMing (acting as game master for) a game called Dark Heresy. Dark Heresy is a tabletop RPG set in the Warhammer 40k universe. For those unfamiliar with the setting, Warhammer 40k is a science fiction setting in the year 40,000, and the future is not nice. In fact, it's termed "grim, dark" in the official book descriptions, and "grimdark" has become a minor meme around the internet to describe the gritty, harsh conditions that characterize this setting.
Rather than attempt to describe the setting further, I'll give you the official description:
"It is the 41st Millennium. For more than a hundred centuries the Emperor of Mankind has sat immobile on the Golden Throne of Earth. He is the master of mankind by the will of the gods and master of a million worlds by the might of his inexhaustible armies. He is a rotting carcass writhing invisibly with power from the Dark Age of Technology. He is the Carrion Lord of the vast Imperium of Man for whom a thousand souls are sacrificed every day so that he may never truly die.Yet even in his deathless state, the Emperor continues his eternal vigilance. Mighty battlefleets cross the daemon-infested miasma of the Warp, the only route between distant stars, their way lit by the Astronomican, the psychic manifestation of the Emperor's will. Vast armies give battle in His name on uncounted worlds. Greatest amongst his soldiers are the Adeptus Astartes, the Space Marines, bio-engineered super-warriors. Their comrades in arms are legion: the Imperial Guard and countless planetary defense forces, the ever-vigilant Inquisition and the tech-priests of the Adeptus Mechanicus to name only a few. But for all their multitudes, they are barely enough to hold off the ever-present threat to humanity from aliens, heretics, mutants -- and far, far worse. To be a man in such times is to be one amongst untold billions. It is to live in the cruelest and most bloody regime imaginable. These are the tales of those times. Forget the power of technology and science, for so much has been forgotten, never to be relearned. Forget the promise of progress and understanding, for in the grim dark future there is only war. There is no peace amongst the stars, only an eternity of carnage and slaughter, and the laughter of thirsting gods."Basically, the human regime is harsh, xenophobic, and violently intolerant. The average human can expect a short life filled with little more than work and duty, often cut violently short. But the alternative to the Imperium is much worse - chaos powers and violent aliens are barely kept at bay by Imperial armies.
I've read several books set in this universe - they can be very good, but also very bad, as books commissioned and controlled by a company can be (other examples include Dragonlance, Forgotten Realms, Star Wars, Star Trek, and others). If the setting sounds interesting and you'd like to try some of the novels, I'd highly recommend the Last Chancers series, or for a lighter and more fun introduction, the Ciaphus Cain series.
I've been running the basic, vanilla game. However, there are many supplemental sourcebooks. Additionally, Games Workshop (the Warhammer publisher) has released two standalone expansions to the game called Rogue Trader and Deathwatch. In Rogue Trader, rather than playing Acolytes in the Inquisition, the characters are rogue traders in a ship out to seek their fortune. A rogue trader is a combination of freelance explorer, conquistador and merchant. They are Imperial servants, given a ship, a crew, a contingent of Marines or Guardsmen and carte blanche to roam the worlds beyond Imperial control. Rogue Trader uses a different system for wealth than Dark Heresy, and in general the players have access to more stuff right off the bat since they are more autonomous and have their own ship.
I'm currently running the introductory adventure in Dark Heresy for my local gaming group. I'm also considering running a play by post version with my guild. Finally, I'm hoping to join a Dark Heresy game next month as a player. I'll update here with how each have gone, perhaps including the full play by post if we complete it!