WDT (Web Developer Tools) is a suite of many different tools for web developers. Not all of them are useful to me but they might be to you… ;-)
It includes an HTTP header viewer, an HTML/CSS inspector, a JavaScript console, a cookie manager, and more. All packed into a single app.
For instance, I find it useful how quickly you can inspect the HTTP headers of a page to debug redirects or caching issues. It is the kind of tool that saves you from opening Firebug or Chrome DevTools for quick checks.
Give it a try if you do any web development! :-)