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.
HTTP Header Live - Displays the HTTP header. Edit it and send it.
Google Chrome Developer Tools - Get started with Google Chrome's built-in web developer tools.
RequestHeaders.com - HTTP Header Checker Tools
TrueMetrics Analytics Inspector - Analytics Inspector provides an easy way to validate web analytics and marketing integration, as well as offers a great way to quickly validate the instrumentation via the unique and simplified user interface.