I’m still surprised that some of my serious web developer friends and colleagues don’t know about this one.

The Web Developer Toolbar plugs into Firefox, Flock, SeaMonkey, and Chrome, and works on Windows, Mac OSX, and Linux. This little extension has been an amazingly valuable debugging tool for just about any website related-problem you could dream of. Image errors? Got you covered. Can’t find the div? Found! What font size is that H2? Identified! Anyway, you get the idea.