Screeps is a fun game and has a unique angle to it. While it still has some room to grow, it's moving in the right direction. Using actual programming instead of a pseudo-language actually allows the players to focus on better coding habits and makes practice fun. There is a very active community of people on the game's Slack channel so it also has a touch of the MMO chat to it for the social programmers out there.
[Trailer] Screeps - MMO sandbox strategy game for programmers