Firefox Developer Tools - Examine, edit, and debug HTML, CSS, and JavaScript on the desktop and on mobile.
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.
Sky Live HTTP Headers - Sky Live HTTP Headers is an open-source browser extension that visually displays HTTP requests and response headers from every website you visit.
Code Cola - Code Cola is a chrome extension to editor online pages' style visibility.
Modify Header Value (HTTP Headers) - Modify Header Value (HTTP Headers) is a browser extension that allows you to add, modify or remove a header for any request to your preferred domains.