The box of free space that I was given a moment ago is not enough to express my passion for this game. Arma stands in a league of its own. It's a military sandbox simulator that provides you with an enormous amount of tools to create whatever you like. A huge scale military scenario that you control in real-time under RTS-like game mode with real players on the ground? A story-driven exploration scenario where you've created everything yourself, from props & uniforms to a massive (or not) terrain? Or perhaps a CIA-like covert operation? Everything you need to know is in their community wiki. You do you, while Arma provides you means.