Godot Engine - Feature-packed 2D and 3D open source game engine.
Blitz3D - Create 2D and 3D Games for Windows.
HaxeFlixel - Create cross-platform games easier and free. All with one codebase.
BlitzPlus - Create 2D Games for Windows.
Orx - Orx - a portable open source game engine
Monkey 2 - An easy to use, cross platform, games oriented programming language