Terraria - Dig, Fight, Build! The very world is at your fingertips as you fight for survival, fortune, and glory.
libtwo - All-purpose c++ app prototyping library, focused towards live graphical apps and games
Minecraft - A block-building game that allows you to create and explore entire worlds from scratch.
Grit Game Engine - The Grit Game Engine is an MIT-Licensed Open Source Game Engine that is highly scriptable via Lua.
Roblox - Reimagining the way people come together.
MOS - Lightweight game engine.