Garry's Mod - Sandbox based on Source. This game was developed by Garry Newman, the same one who developed RUST, for example. The game was released in 2010, having found its Steam Page as a regular mod for Half Life 2. At the moment, Garry's Mod is not only a regular sandbox, but also a tool for creating images ("Posters" in the GMOD community), and a full-fledged platform for user servers.