SWEngine (2D Game Engine)

SWEngine is a powerful and free 2D Game Engine. It has simple and flexible API to easily develop 2D games. It uses OpenGL, Glew, GLSL, FMod, Box2D and DirectInput in it's framework.
It also has a good API doc and lots of tutorials (nearly 40). You can access SWEngine and other SWWorkshop products at http://www.skywar.org