Green Ronin Publishing generic system RPG which comes with four settings but usable with a wide variety of settings that can be adapted by the GM. From the back of the book:
From the publishers website:
Worlds of imagination and adventure await you!
With True20 Adventure Roleplaying you embark on journeys of the imagination, with heroes and stories you and your friends create. This book tells you everything you need to know to play an exciting adventure roleplaying game: how to create your own heroes, how to handle action and conflict, how to narrate adventures, and how to keep it all fair and fun.
True20 simplifies and streamlines the adventure roleplaying experience: everything is resolved using the same core system and all you need is a single 20-sided die to play! There's no bookkeeping of points, whether hit points, power points, or experience points. Game play is fast and furious thanks to innovations like the Toughness save damage system, based on the award-winning Mutants & Masterminds RPG. You can use True20 to model nearly any genre, from fantasy and historical to modern and scifi. The winners of Green Ronin's True20 Setting Search demonstrate this flexibility. These four, ready-to-play settings show the strength and adaptability of True20.
Plunge yourself into these fantastic worlds of adventure!
Soar on a magic carpet over minarets and domes, slip past the guards of the sultan's palace, and bargain with cunning and treacherous djinn in Caliphate Nights.
Sail the sea of stars past the great maelstrom to fantastic planets inhabited by a mix of alien races and humans from a dying Earth in Lux Aeternum.
Climb into the cockpit of your Tetsujin armored warrior to battle giant radiation-spawned monsters and keep your nation safe in Mecha vs. Kaiju.
Learn the secrets of the past, and how to manipulate the future, of the hidden struggles occurring in between ticks of the clock in Borrowed Time.
All this and more awaits you. If you want a fast, simple, and fun game system that's still familiar to players of the world's most popular RPGs, if you want a vehicle for traveling to new worlds of adventure, then True20 is for you! True fun, true excitement, true adventure...