MooTools is a compact, modular, Object-Oriented JavaScript framework designed for the intermediate to advanced JavaScript developer. It allows you to write powerful, flexible, and cross-browser code with its elegant, well documented, and coherent API.

Dojo saves you time and scales with your development process, using web standards as its platform. It’s the toolkit experienced developers turn to for building high quality desktop and mobile web applications.
From simple websites to large packaged enterprise applications whether desktop or mobile, Dojo will meet your needs.

Easy Ajax and DOM manipulation for dynamic web applications

sIFR is meant to replace short passages of plain browser text with text rendered in your typeface of choice, regardless of whether or not your users have that font installed on their systems. It accomplishes this by using a combination of javascript, CSS, and Flash

script.aculo.us provides you with
easy-to-use, cross-browser user
interface JavaScript libraries to make
your web sites and web applications fly.

The Write Less, Do More, JavaScript Library

Greasemonkey is a Firefox extension that allows you to customize the way webpages look and function.