ViewHTTPHeaders.com - View HTTP header response for any web page.
Firefox Developer Tools - Examine, edit, and debug HTML, CSS, and JavaScript on the desktop and on mobile.
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.
Google Chrome Developer Tools - Get started with Google Chrome's built-in web developer tools.
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.
Fiddler - Fiddler is a debugging program for websites.