With the new release of JQuery come a lot of new features and goodies. Hers a great run down of whats new: http://net.tutsplus.com/tutorials/javascript-ajax/jquery-1-4-released-the-15-new-features-you-must-know/

Raphaël is a newly released JS library which does some pretty amazing stuff with vector graphics.

Read about it on it’s official site:
http://raphaeljs.com/

Also, here’s a friendly person with a Russian-sounding accent giving an overview of it.

Raphaël Tutorial—Basics from Dmitry Baranovskiy on Vimeo.

Cracking logo

A great collection of jQuery powered widgets and special effects. And free to use too!
jQuery Interface Elements

Some very clever javascript/css image manipulation effects.

Allows you to apply refelctions, bevels, page corners and all sorts.