Cheat Engine is an open source tool designed to help you with modifying single player games running under windows 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 useful 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.
But it can also be used for other tweaks. It gives you the entire memory to edit.
Speedhack: This is one of the most useful features. You don’t understand the memory it displays, no problem use the Enable speedhack and see the difference to which ever application you want to apply. [May not always work, if not used cautiously the application may crash]