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.
A small tutorial on how to build a password strength meter like the one on Google's new account form.