Firefox Developer Tools - Examine, edit, and debug HTML, CSS, and JavaScript on the desktop and on mobile.
ViewHTTPHeaders.com - View HTTP header response for any web page.
Google Chrome Developer Tools - Get started with Google Chrome's built-in web developer tools.
HTTP Header Live - Displays the HTTP header. Edit it and send it.
Code Cola - Code Cola is a chrome extension to editor online pages' style visibility.
RequestHeaders.com - HTTP Header Checker Tools