Besides form controls, alerts & similar simple JavaScript usage, you can almost do anything with the help of JavaScript (& Canvas). Some samples are: * Drag’n drops * Animations * CSS manipulations, Ajax & more.
Scriptaculous, MooTools, jQuery and much more: all you need to develop modern web interfaces and add new features to your website.
With typeface.js you can embed custom fonts in your web pages so you don't have to render text to images.
SoundManager 2 is a Javascript Sound API which talks to Flash, effectively mapping most of Flash 8's native sound capabilities to Javascript. It enables web developers and front-end engineers to programmatically control sound in a cross-browser/platform
Note that Firefox 2.0.0.11, IE 7, and Safari 3.0.3 beta all get some of the regex wrong.
A small tutorial on how to build a password strength meter like the one on Google's new account form.