Zero Protocol takes inspiration from classic sci-fi FPS games like Half-Life and System Shock, and you can play it right now on Steam.
Metaphor: ReFantazio, the upcoming JRPG from Persona 5 and Catherine creator Atlus, is already blowing up on Steam thanks to its demo.
Game engines are software frameworks for game development. Game engines do the heavy lifting for developers so they can focus on other aspects of game development.