This is something I downloaded a while back, and use sporadically, but every time I do, I'm amazed.  In a single right-click menu it gives you access to just about any tool a good web developer needs when browsing their app.  Turn off styles/javascript/cache/images/etc for testing.  Switch CSS Media types, edit css on-the-fly.  Outline divs, tables, links, etc.  Validate using a variety of tools- even localhost sites.   The one humble menu item that spurred me to post this, though, is 'Resize To... 800x600'.  Perfect for testing your app on that ginormus monitor.

http://chrispederick.com/work/webdeveloper/