Here's a good source for the basics of Google Web Toolkit. see here

In this article, he discusses the basics and Also touches on JSNI and tips on server-side implementations...
He also touches on the script.aculo.us jscrip library...

The script.aculo.us is a great way to add interactivity to your site and
enhances the users experience through effects.

For more info, visit the script.aculo.us home page and see online demonstration...

There is also a sources that wraps the script.aculo.us and makes it easy to integrate
in GWT... See the GWT Component library.

Other custom widgets are also found here like the calendar widget and the auto-completion Widget.