Firefox Developer Tools - Examine, edit, and debug HTML, CSS, and JavaScript on the desktop and on mobile.
Charles Proxy - HTTP proxy / HTTP monitor / Reverse Proxy
Code Cola - Code Cola is a chrome extension to editor online pages' style visibility.
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.
DOMScope - Easy debugging for mobile web. Remote web inspector with zero configuration.
Postman - The Collaboration Platform for API Development