libtwo - All-purpose c++ app prototyping library, focused towards live graphical apps and games
irrlicht - This caused the Irrlicht homepage and forum to work badly. Over the last days most things started working again – except the forum continues to be broken. Old posts can be read but everything else is currently not working.
Grit Game Engine - The Grit Game Engine is an MIT-Licensed Open Source Game Engine that is highly scriptable via Lua.
Unity - The multiplatform game creation tools for everyone.
MOS - Lightweight game engine.
Godot Engine - Feature-packed 2D and 3D open source game engine.