Although the entire suite of tools is available under a community source license, Zimbra chose to spin off its Ajax toolkit in a separate package called Kabuki that’s licensed under either Apache or Mozilla. The toolkit includes some of the practical tools and widgets but not all of the pieces found in the larger package.
That doesn’t mean Zimbra’s toolkit comes up short, though: Kabuki has an excellent editor, a nice tree package, and a layout mechanism that can be used to build sophisticated user interfaces.
You can fix some of this with the right style sheets and some good design, but it’s going to take some extra programming effort. If you’re willing to take that on, the Zimbra AJAX mechanism is still filled with largely practical tools.
Google Web Toolkit
The breadth of the toolkit is solid. Google has built in many of the standard widgets but hasn’t included some of the flashier mechanisms for animation.
I found Google’s system surprisingly easy to use — it ended up being close to working with a fancy version of the AWT (Abstract Window Toolkit) or a simpler version of Swing. The package is designed to build a full application that runs inside the browser, not just add a bit of zip to your flat pages.