mitmproxy : mitmproxy is your swiss-army knife for debugging, testing, privacy measurements, and penetration testing. It can be used to intercept, inspect, modify...
httpbin.org : A simple HTTP Request & Response Service.
Zombie.js : Zombie.js is a lightweight framework for testing client-side JavaScript code in a simulated environment. No browser required.
Polly.JS : Polly.JS is a standalone, framework-agnostic JavaScript library that enables recording, replaying, and stubbing HTTP interactions.
Polly taps into nat...
HttpWatch : An HTTP Viewer and HTTP Sniffer for IE and Firefox
toxy is a fully programmatic and hackable HTTP proxy to simulate server failure scenarios and unexpected network conditions, built for node.js/io.js.