Orx game engine
Orx is a pretty neat little 2d game engine I found recently after embarking upon a little quest to procrastinate on schoolwork by downloading and playing with open source game/graphics engines. Orx has a C interface and so it forces you to write nice code.
I tried to compile it on OS X 10.9 initially but ran into trouble. So then I changed the premake4.lua files a bit and got it to work (also had to fix some weird glext.h typedef redefinition conflict). I put up the patch here, and here's a picture from a demo: