Synthetic World is an open world sandbox game. It allows you via a seed to create a unique landscape with Mountains, Oceans, Trees and Beachs. Although it looks like a normal game engine, everything in the game is based on voxels. So everything you see can be changed. You can cut down trees, build a house or dig into the ground!
Official Website : http://www.staudsoft.com/
Status : In development, Playable
Platform : Windows, Linux
Multiplayer : Yes
Price : Free