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ViewHTTPHeaders.com - View HTTP header response for any web page.
HTTP Toolkit - Beautiful, cross-platform & open-source tools to debug, test & build with HTTP(S). One-click setup for browsers, servers, Android, CLI tools, scripts and more.
HTTP Header Live - Displays the HTTP header. Edit it and send it.
RequestHeaders.com - HTTP Header Checker Tools
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.