HttpWatch : An HTTP Viewer and HTTP Sniffer for IE and Firefox
Wireshark : 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 fac...
Sitespeed.io : Sitespeed.io is an open source tool that helps you analyze and optimize your website speed and performance based on performance best practices. It col...
mitmproxy : mitmproxy is your swiss-army knife for debugging, testing, privacy measurements, and penetration testing. It can be used to intercept, inspect, modify...
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 functional tests or even on a real deployment through the command-line.