HTTP Debugger - Debug HTTP API calls to a back-end and between back-ends. Easy of use, clean UI, and short ramp-up time. Not a proxy, no network issues!
Wireshark - Wireshark is a network protocol analyzer for Unix and Windows. It lets you capture and interactively browse the traffic running on a computer network.
Google Chrome Developer Tools - Get started with Google Chrome's built-in web developer tools.
tcpdump - tcpdump is a common packet analyzer that runs under the command line.
Firefox Developer Tools - Examine, edit, and debug HTML, CSS, and JavaScript on the desktop and on mobile.
Ettercap - Ettercap is a suite for man in the middle attacks on LAN.