Here’s one open source game engine for iOS4 that continues to impress me with its rich features and abilities. Galaxy Engine is a complete game development package for Mac, iPhone, and iPad. It is programmed in Cocoa and uses the OpenGL, OpenGL ES, and OpenAL libraries to give you a powerful core with direct access to iPhone SDK features such as Game Center. Galaxy Engine is designed with indie developers in mind. Which means that it is designed to be as efficient and flexible as possible.