Willpower

The One Stat/One Die RPG System From the introduction: Willpower is very simple RPG. All you need to play is a Game Master (GM) some players and a D6. You can play this game in any setting including fantasy, modern or sci-fi. In this system the player's characters can do anything that they have enough will to do. A mother can lift a car, but only if her child is under it. A ranger can shoot two orcs with one arrow, but only if his friend is in danger.  The characters only have the will to make an action when they believe that they can do a thing.