Sandbox
A place to test and try and investigate.

From Dust
0.0.0.23 (23 May 2012)From Dust is the award winning game where you control Nature and master the elements for the survival of your people.
Genre: Simulation 
Terasology
1.x (14 Mar 2016)Terasology is an open-source survival and discovery game with influences from Minecraft, Dwarf Fortress, and Dungeon Keeper.
Genre: Role-playing 
Harvest: Massive Encounter
1.18 (25 Apr 2012)They looked for love and understanding.
Genre: Strategy 
Island Forge
Island Forge is the brand-new (yet old-school) storytelling MMORPG that's all about player-created content.
Genre: MMO, Social, Role-playing 
Epic Inventor
by Pixel Prone
1.0
(30 Apr 2012)
Epic Inventor combines side-scroller, RPG and RTS elements into one random ball of Epic!
Genre: Arcade, Platform, Strategy, Role-playing 
Golems
0.57.0 (31 Jan 2011)Golems is a 3D recreational physics simulator
Build robots, spaceships, clocks, catapults or cannons.
Genre: Simulation 
Mythruna
Build 2012-05-08 (8 May 2012)Mythruna is a game under construction combining extensive world building, crafting, and sandbox elements with just a touch of role-playing.
Genre: Role-playing 
Tales of Maj'Eyal
by DarkGod
1.2.5
(4 Nov 2014)
Tales of Maj'Eyal (ToME) is an open-source, single-player, tactical role-playing roguelike and action game set in the world of Eyal.
Genre: Roguelike, Role-playing 
Script Blocks
Alpha 13 (8 Aug 2011)Script Blocks is a block building game.
Genre: Puzzle, Simulation 
Minetest Game
0.4.16 (3 Jun 2017)Minetest Game is a simple and peaceful “sandbox game”, this means there are no goals and no built-in enemies.
Genre: Role-playing 
King Arthur's Gold
King Arthur’s Gold is a game about mining, building castles and destroying your enemies.
Genre: Platform, Strategy