No Myth

From the introduction:

This game is designed to facilitate a collaborative style of short-form storytelling. The premise, and the reason it's called No Myth, is this: few things exist outside of that which the players create. No Myth does not have a single player controlling the setting and non-player characters. This role (called the GM’s role in a traditional RPG) will be divided between everyone at the table.

The game focuses on sandbox-style play, where the player characters have a lot of options and can explore at will. The goal is to allow sandbox play in a more improvised style, without quite as much preparation.