Charles proxy, Paros, Burp and Fiddler are all great tools to intercept and modify traffic.

Great for debugging or just inspecting/reversing/hacking in general.

Im glad if similar tools are being made available as browser extensions, it might lower the barrier to entry and get more people poking at the network layer.

PS: https://github.com/square/PonyDebugger is a cool debugger that lets you use Chrome developer tools when developing iOS apps.

PS2: https://paw.cloud/ and some other tools take these proxies to a whole new level in terms of UI/polish.