Charles Proxy - HTTP proxy / HTTP monitor / Reverse Proxy
HTTPDump - A simple, beautiful Webhook debugging tool.
Wireshark - Wireshark is a network protocol analyzer for Unix and Windows. It lets you capture and interactively browse the traffic running on a computer network.
httpbin(1) - HTTP request and response service
Postman - The Collaboration Platform for API Development
Proxyman.io - Proxyman is a high-performance macOS app, which enables developers to view HTTP/HTTPS requests from apps and domains.