Ayman Hourieh covers some interesting manipulation of Arrays that accommodates typical data structures. He also covers how to bind methods to objects, sorting with custom functions. He shows a custom AssertException, and covers using Firebug for debugging. He covers some JavaScript quirks: static local variables, null, undefined and delete.
Based on Simon Willison's original 2005 eTech talk, Simon expands this into a full article covering the expressive functionality of JavaScript. He covers the basics of JavaScript: literals, variables, functions, scope, control flow, objects, classes, inner functions and closures. Everything a developer needs to know before diving into more advanced JavaScript.
Erik Arvidsson warns against extending the JavaScript Object with Object.prototype, as it can lead to breakages in third party code when they use the for(key in obj) method of iterating through any class (since they all are subclasses of Object). Instead Erik recommends extending the classes that directly benefit from the extending functions, and treat Object as a final constant. Erik also suggests that hashes and associative arrays should be done in JavaScript using Object, not Arrays.