ZK is my favorite Web 2.0 toolkit. In fact, it's so powerful that I'd call it, a Web 3.0 toolkit. A true Web 3.0 toolkit where you do not have to write a single line of Javascript, which is compatible with all major Web browsers, and which allows you to write glue code in major scripting languages (Ruby, Python, Groovy, etc) even if the framework is based on Java.

In this nice article, you'll find a nice comparison between GWT and ZK. It's worth a read !

P.S.: ZK is also in the Top 10 most active projects on Sourceforge since 18 months !