http://www.godotengine.org/wp/godot open source game engine
open sourced under an MIT license, it's completely free. it supports Windows, Mac OSX, and Linux. additionally, you can export games to html 5 or playstation 3/vita.
it can use c++ but has it's own scripting language based on python.
it's great for 2d games and lightweight 3d games, but don't expect forgetin' frostbite engine quality from this engine.