Many developers would have heard of Adobe Flex. It makes use of MXML to create a UI and ActionScript to do the event/actions responses. Things didn't stop there ,OpenLaszlo came up with LZX, which is declarative XML description of an application interface. I admire the OpenLaszlo library because it gives the facility to compile its code to Flash and DHTML. These two products are much mature, but there are other libraries to lookout for.
Google too is in pursuit of Declarative XML structure for defining GWT widgets. The project is in still incubator. But this framework will be only for GWT. When compared to Apache’s XAP, DeclarativeUi is not generic and have less features.
Apache and Google may be very well known, but I found a hidden treasure called ZK. ZK claims to be yet another “#1 Ajax and mobile framework”. ZK makes use of off-the-shelf XUL and XHTML components, and manipulate them by listening to events triggered by users. ZK is available for Mobiles also, so applications will work on any devices that support J2ME, without any modification to applications. ZK has come up with the ZUML which is based on XUL. This makes it more generic and is highly integrated with Java. The framework provides a huge set of widgets and functionality.ZK framework's architecture is explained here.