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...
Diagnostic.css : Diagnostic.css is a stylesheet which allows the user to test for common errors in a page's markup.
Also : https://github.com/karlgroves/diagnostic.css
mock : A Python Mocking and Patching Library for Testing
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.