Fiddler - Fiddler is a debugging program for websites.
ViewHTTPHeaders.com - View HTTP header response for any web page.
Proxyman.io - Proxyman is a high-performance macOS app, which enables developers to view HTTP/HTTPS requests from apps and domains.
HTTP Header Live - Displays the HTTP header. Edit it and send it.
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.
RequestHeaders.com - HTTP Header Checker Tools