GWT - Google Web Toolkit
Extract from GWT.
With Google Web Toolkit (GWT), you write your AJAX front-end in the Java
that automatically works across all major browsers. During development, you can
iterate quickly in the same "edit - refesh - view" cycle you're accustomed to
your Java code line by line. When you're ready to deploy, GWT compiles your Java
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.
Also, there is a GWT-Ext Widget library available. It provides many UI extension. Pretty exciting.