WebSniffer - WebSniffer is an online tool to outlook HTTP request and response headers.
Google Chrome Developer Tools - Get started with Google Chrome's built-in web developer tools.
Smart Header - Smart Header is a browser extension that lets you manipulate the HTTP request headers and response header automatically.
Firefox Developer Tools - Examine, edit, and debug HTML, CSS, and JavaScript on the desktop and on mobile.
HTTP Header Spy - HTTP Header Spy is a browser extension that allows you to inspect the request and response header of any website URL.
Postman - The Collaboration Platform for API Development