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.
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.
For new users it is recommended to go through the tutorial(The one that comes with Cheat Engine, you can find it in your programs list after installing) and at least reach step 5 for basic understanding of the usage of Cheat Engine
Charles is a web proxy (HTTP Proxy / HTTP Monitor) that runs on your own computer. Your web browser (or any other Internet application) is then configured to access the Internet through Charles, and Charles is then able to record and display for you all of the data that is sent and received.
In Web and Internet development you are unable to see what is being sent and received between your web browser / client and the server. Without this visibility it is difficult and time-consuming to determine exactly where the fault is. Charles makes it easy to see what is happening, so you can quickly diagnose and fix problems.
Charles makes debugging quick, reliable and advanced; saving you time and frustration!
Fiddler is an HTTP debugging proxy server application written by Eric Lawrence of Microsoft. It captures HTTP traffic and logs it for the user to review. It can also be used to "fiddle" with HTTP traffic as it is being sent. By default, traffic from Microsoft's WinINET HTTP(S) stack is automatically directed through Fiddler at runtime, but any browser or application can be configured to route traffic through Fiddler.