Terraria - Dig, Fight, Build! The very world is at your fingertips as you fight for survival, fortune, and glory.
Lunacraft - Lunacraft focuses on Endless Creativity and Exploration elements.
SimPhy - Interactive 2D & 3D Physics simulation software
Roblox - Reimagining the way people come together.
Physion - Physics Simulation Sandbox
Blockland - Blockland is a non-competitive multiplayer sandbox-style construction game. It's kind of like playing with legos on the internet.