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...
Cypress : JavaScript End to End Testing Framework. Fast, easy and reliable testing for anything that runs in a browser.
toxy : toxy is a fully programmatic and hackable HTTP proxy to simulate server failure scenarios and unexpected network conditions, built for node.js/io.js.
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.