Cheat Engine is an open source tool designed to help you with modifying single player games running under window so you can make them harder or easier depending on your preference(e.g: Find that 100hp is too easy, try playing a game with a max of 1 HP), but also contains other usefull tools to help debugging games and even normal applications.

"Raph Koster points us to a useful piece of kit: OGLE, the OpenGL Extractor, is a tool for extracting 3D models from OpenGL applications. By sitting between the application and the OpenGL library, OGLE can effectively take 3D screenshots--and the possibilities for gaming are immense. "