Alternity is a science fiction role-playing game (RPG) published by TSR in 1998. Following the acquisition of TSR by Wizards of the Coast, the game was discontinued in 2000 as part of a broader rationalisation of TSR's business holdings, but it retains a small but devoted fanbase.
From the Player's Handbook:
The Alternity game is TSR's roleplaying game of modern action to far-future adventure. It's science fiction, in so much that it draws upon the speculative storytelling style that deals with the future, space and scientific breakthroughs yet to be made. It's a game of "what ifs."
What if... humanity develops a means to travel among the stars? What if... We aren't alone in the universe? What if... the world we know ends tomorrow? What if... alien entities invade our world?
No matter what supposition or speculative leap of imaginiation you come up with, as long as it is founded in a scientific (or quasiscientific) theory that's internally consistent, you're in the realm of science fiction. From a modern world that looks like today but features a technological twist that makes it a very different place (like a new weapon, computer or other device), to a far-future era where humanity has moved to other worlds, any setting and any technology is easily adapted for play.
The Alternity game, then, is a game of unlimited possibilities, exciting potentials, and imaginative "what ifs." The game system allows for may styles of play, though it never lets you lose sight of the fun of roleplaying. The rules are presented in the time-tested format that has made the Advanced Dungeons & Dragons game the best-selling troleplaying game of all time - two hardcover rules books, one for game referees (called Gamemasters in the Alternity game), that combine to create a complete and dynamic roleplaying game."
Source: Wikipedia, "Alternity", available under the CC-BY-SA License.