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, but also contains other usefull tools to help debugging games and even normal applications.

It comes with a memory scanner to quickly scan for variables used within a game and allow you to change them, but it also comes with a debugger, disassembler, assembler, speedhack, trainer maker, direct 3D manipulation tools, system inspection tools and more.