Buildcraft is an infinite-world block sandbox game. Buildcraft with its truly multi-player feature, you can create
world as you like.
Walk around, dig and build in an infinite voxel world, and craft stuff from raw materials to help you along the way;
Simplistic Mudding API that supports many kinds of additions and modifications to the game;
Multiplayer support for tens of players, via servers or local WIFI;
Voxel based lighting with gameplay consequences (light caves and buildings with torches);
Almost infinite world and a beautiful map generator.