The game, Minecraft, is quite possibly the only graphical multiplayer game that captures the text-based MUD experience of years past. By that, I mean that not only can you login and play on a server with other people, but you can also download the server software and host your own game server allowing others to connect and play. Additionally, with some mods or some coding, you can add your own features and functions to the game.
Currently, Notch, the creator of Minecraft, does not prefer to have people creating mods and hacking his code, but he has begun work on an API that will allow people to build upon the base game.
Aside from that, even the single-player version of the game has captured my imagination because of the simplicity and the balance between risk and reward. You feel a sense of accomplishment when you build something out of the resources that you’ve collected. You feel a great loss when a mob kills you causing you to drop everything you were carrying. You are challenged after dying to find your way back from your spawn point to gather your dropped belongings and to find your home that you’ve built.
The world is infinitely expanding and procedurally generated. Minecraft is an excellent sandbox game which is evolving to be one of the best online games I’ve played.