TUG is a fantasy sandbox role playing game with focus on crafting & building, exploration and adventure. It is based on a non-blocky voxel engine and comes with three options: single-player mode, private server for you and your friends and - a future option - the public, dedicated server with 1000 player capacity. It features three game modes:
Proving Grounds (PvP Arena/Battle Royal, not implemented yet as of May 2015).
Modding is a fundamental part of TUG so pretty much anything the TUG developers can do the players who host servers can do. As of May 2015 TUG is in Early Access/Development but can be played if an Early Access key is bought.
TUG is developed by Nerd Kingdom, a collaboration of "social scientists, economists, artists, and developers" and based on "ongoing sociology and behavioral science research of player interactions within virtual worlds" (http://nerdkingdomofficial.tumblr.com/).