A clever hands-on interactive mini-course by Nathan Whitehead that guides you through bite-sized chunks from variables, to functions, to return values, to functions as first-class objects and right into closures and finally continuations. The hands-on exercises are JS Linted and unit-tested which gives you ample space to try out your new-found knowledge.
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).
Exploring the features of Firebug, with code examples covering using the console, timing code, stack traces, debugging and breakpoints, profiling, tracing HTTP requests. An excellent overview of Firebug's functionality
A port of Douglas Crockford's command line JSMin in Perl. It removes extraneous whitespace and comments, but does not obfuscate the code.
A collection of animated 'in progress' graphics for use in Ajax applications. Includes clock-type indicators and progress bars.
Three top features of jQuery: the multitude of selectors, manipulating attributes, and tacking Ajax/JSON requests. Choses jQuery over Prototype
Christian Heilmann describes an approach to building complex web application by basing them around events, particularly around YUI's CustomEvent class.
Svend Tofte puts together a detailed guide to setting up and using the Venkman debugger. From basic debugging to watchlists and call stacks, breakpoints, and more advanced techniques like meta comments and profiling. It also contains a list of other useful Venkman resources.