netcat - Netcat is a featured networking utility which reads and writes data across network connections...

Charles Proxy - HTTP proxy / HTTP monitor / Reverse Proxy

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.

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.

SmartSniff - SmartSniff is a packet sniffer that capture TCP/IP packets and display them as sequence of conversations between clients and servers.

Weer - A HTTP protocol debugger with Chrome DevTools frontend interface