ZK is an open-source web development framework that enables web applications to have the rich user experiences and low development costs that desktop applications have had for years. ZK includes an Ajax-based event-driven engine, rich sets of XML User Interface Language (XUL) and XHTML components, and a markup language.
http://rapidshare.com/files/183172268/Packt.Publishing.ZK.Developers.Guide.Dec.2007.pdf