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...
HttpWatch : An HTTP Viewer and HTTP Sniffer for IE and Firefox
OWASP Mutillidae II : OWASP Mutillidae II is a free, open source, deliberately vulnerable web-application providing a target for web-security enthusiest. Mutillidae can be ...
Mock Service Worker : Mock by intercepting requests on the network level. Seamlessly reuse the same mock definition for testing, development, and debugging.
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.