Google Web Toolkit is an open source Java development framework that can be used to build web applications using AJAX easily and you do not have to worry about issues like cross-browser compatibility.
GWT Major Features:
- Real-time debugging
- Cross-browser compatibility
- Reusable UI Components
- Simplified DOM API
- Completely Open Source
Also, during development you can run and debug your application in hosted mode by running it as Java Bytecode within JVM using GWT's Hosted Web Browser.
To get an overview of the its architecture, please see this
On a downside, there are few issues still unresolved:
- GWT 1.4 does not support JDK 1.5 version but GWT 1.5 Release Candidate do support it
If you haven't tried it yet, download it right now here and have fun making easy to use and interactive good looking web applications.