HttpWatch : An HTTP Viewer and HTTP Sniffer for IE and Firefox
Vaurien : Vaurien is basically a Chaos Monkey for your TCP connections. Vaurien acts as a proxy between your application and any backend.
You can use it in your...
Postman : Save requests and organize your APIs into Collections. Create and send any HTTP request using the awesome Postman request builder. Write your own test...
LibreNMS : A fully featured network monitoring system that provides a wealth of features and device support.
Wireshark is the world's foremost network protocol analyzer. It lets you see what's happening on your network at a microscopic level. It is the de facto (and often de jure) standard across many industries and educational institutions.