Screeps is the "world's first
MMO strategy open world game for programmers".
You control creeps which can perform various actions, and the goal is to create your own empire and to conquer the territories of other players. Depending on how you program your creeps, they can react to events while you are offline.
The MMO (with a single persistent world) starts next week as "early preview", the duel mode (1 vs. 1) is in development, and the arena mode temporarily disabled, so currently you can only play the "Tutorial", the "Survival" mode (you against NPCs), and the "Custom Mode" (for testing).
For questions about the gameplay, you can head to Arqade, and for questions about the programming to play it, you can head to Stack Overflow.