Screeps: World is an open source MMO RTS sandbox game for programming enthusiasts, wherein the core mechanic is programming your units AI. You control your colony by writing JavaScript which operates 24/7 in the single persistent open world filled by other players on par with you.
Screeps means "scripting creeps." You use real JavaScript to script behavior of your units, or creeps. The Screeps franchise is aimed at programming enthusiasts and consists of two games: World and Arena.