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.
ViewHTTPHeaders.com - View HTTP header response for any web page.
WebSniffer - WebSniffer is an online tool to outlook HTTP request and response headers.
HTTP Debugger - Debug HTTP API calls to a back-end and between back-ends. Easy of use, clean UI, and short ramp-up time. Not a proxy, no network issues!
Fiddler - Fiddler is a debugging program for websites.
Google Chrome Developer Tools - Get started with Google Chrome's built-in web developer tools.