netcat - Netcat is a featured networking utility which reads and writes data across network connections...
Charles Proxy - HTTP proxy / HTTP monitor / Reverse Proxy
PortMapper - Do you need to map a specific TCP or UDP port on your machine to another port on another machine?
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.
Komodia Relay - Free open source tool to perform TCP based session relay
Postman - The Collaboration Platform for API Development