Minecraft - A block-building game that allows you to create and explore entire worlds from scratch.
Sandboxels - Sandboxels is an in-browser falling sand simulation game, with mechanics such as heat simulation, electricity, density, chemical reactions, fire, explosions, and over 500 unique elements to play with.
Terraria - Dig, Fight, Build! The very world is at your fingertips as you fight for survival, fortune, and glory.
The Powder Toy - The Powder Toy is a desktop version of the classic 'falling sand' physics sandbox game, it simulates air pressure and velocity as well as heat!
Roblox - Reimagining the way people come together.
iPhysics Sandbox - iPhysics Sandbox is a simple physics sandbox that can be configured to run as a simple game or as a virtual physics lab for STEM education.