Google Web Toolkit (GWT) is a Java software development framework that makes writing AJAX applications easy. With GWT, you can develop and debug AJAX applications in the Java language using the Java development tools of your choice. When you deploy your application to production, the GWT compiler translates your Java application to browser-compliant JavaScript and HTML.
My favorite feature is the built-in RPC support. Instead of writing special methods to parse JSON literals and wrap them in your own objects, you just need to annotate the classes you want to send across the wire, and GWT automagically handles serialization and deserialization for you. The list of features is impressive, but you really have try it out for yourself to appreciate how cool the product is.
If you are at JavaOne, make sure you come see our tech lead, Bruce Johnson, in his session on GWT tomorrow, May 17, at 12:15 pm in the Moscone Center Gateway 102/103.