Charles Proxy - HTTP proxy / HTTP monitor / Reverse Proxy
ViewHTTPHeaders.com - View HTTP header response for any web page.
Weer - A HTTP protocol debugger with Chrome DevTools frontend interface
HTTP Header Live - Displays the HTTP header. Edit it and send it.
Proxyman.io - Proxyman is a high-performance macOS app, which enables developers to view HTTP/HTTPS requests from apps and domains.
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!