Unity - The multiplatform game creation tools for everyone.
libtwo - All-purpose c++ app prototyping library, focused towards live graphical apps and games
LOVE 2D - Hi there! LÖVE is an *awesome* framework you can use to make 2D games in Lua.
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.