libtwo - All-purpose c++ app prototyping library, focused towards live graphical apps and games
Unity - The multiplatform game creation tools for everyone.
Grit Game Engine - The Grit Game Engine is an MIT-Licensed Open Source Game Engine that is highly scriptable via Lua.
Pygame - Pygame is a set of Python modules designed for writing games.
MOS - Lightweight game engine.
GDevelop - GDevelop is an open-source game making software designed to be used by everyone.