Why Hoxy? Tools like Firebug and Chrome developer tools give you the ability to tweak and modify what’s already in the browser, but don’t give you control of what loads into the browser in the first place. Hoxy was built to fill this gap. It lets you alter, in real time, almost any aspect of HTTP traffic. Anything from headers to HTML can be sliced and diced, written or replaced, by Hoxy.
This gives you unprecedented control over how you test HTTP applications, and largely alleviates the need for staging servers that perfectly mimic production. For example, you can use Hoxy to spoof the latest version of jQuery into production, then browse the site and see if anything breaks. Or you can use it to test caching header configurations, as if they’d been implemented in production.
Hoxy @ Github has more detailed documentation, including an architectural overview, system requirements and how-to’s.