HTTP Debugger is a professional HTTP sniffer and analyzer for developers.
You can use HTTP Debugger to debug HTTP API calls to a back-end and between back-ends.
HTTP Debugger is very easy of use, with clean UI, and short ramp-up time.
It's not a proxy, and does not produce network issues!
Charles Proxy - HTTP proxy / HTTP monitor / Reverse Proxy
socat - socat is a relay for bidirectional data transfer between two independent data channels.
Surge for Mac - Advanced Web Debugging Proxy for Mac & iOS
netcat - Netcat is a featured networking utility which reads and writes data across network connections...
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.
Packet Sender - Packet Sender can send and receive UDP, TCP, and SSL on the ports of your choosing.