Terraria - Dig, Fight, Build! The very world is at your fingertips as you fight for survival, fortune, and glory.
Blockland - Blockland is a non-competitive multiplayer sandbox-style construction game. It's kind of like playing with legos on the internet.
SimPhy - Interactive 2D & 3D Physics simulation software
Garry’s Mod - Garry’s Mod is Funny, Sandbox, Co-op, Building, FPS, Action, Single and Multiplayer video game created by Facepunch and published by Valve.
Physion - Physics Simulation Sandbox
Roblox - Reimagining the way people come together.