A cross-browser bookmarklet to examine the box-model of any DOM element on a page. Runs on Internet Explorer, Safari (and Webkit based browsers), Mozilla (including Firefox and Camino).
A Firefox extension from Chris Pederick which adds in a toolbar of web development goodies, from disabling JavaScript, CSS and images through to manipulating cookie information, speed reports and validator tools and error consoles.
Douglas Crockford's presentation on Advanced JavaScript. He covers topics such as inheritance, modules, debugging, efficiency and JSON.