GWT

GWT - Google Web Toolkit

It is very exciting toolkit. Java developer will like it as it can cross-complile Java to JavaScript. Develop in Java, result in JavaScript. Also, it has the most important debugging tool - Step through debugging

Extract from GWT.

With Google Web Toolkit (GWT), you write your AJAX front-end in the Java
programming language which GWT then cross-compiles into optimized JavaScript
that automatically works across all major browsers. During development, you can
iterate quickly in the same "edit - refesh - view" cycle you're accustomed to
with JavaScript, with the added benefit of being able to debug and step through
your Java code line by line. When you're ready to deploy, GWT compiles your Java
source code into optimized, standalone JavaScript files. Easily build one widget
for an existing web page or an entire application using Google Web Toolkit.

Cypal Eclipse Plugin

There is an existing Eclipse plugin for GWT. That is an awesome new for developer who frequently use Eclipse for development.

GWT-Ext

Also, there is a GWT-Ext Widget library available. It provides many UI extension. Pretty exciting.