Beryl XML GUI Library
The Beryl XML GUI library was written to ease the development of graphical user interfaces using Swing on Java. It lets you store user interfaces as XML markup. This will help you avoid unnecessary clutter in your source - Swing code mixed with application logic can become a troublesome and hard to read mess as the application size increases.

jXUL Project
The jXUL project is an open source project to integrate the XUL language into the Java Platform. There are multiple ways in which this integration will take place. In fact, this project will expand as its members find new ways to integrate these two technologies.

Laszlo
Laszlo is an open source platform for the development and delivery of rich Internet applications on the World Wide Web. The Laszlo platform consists of the LZX language and the Laszlo Presentation Server.

Luxor
Luxor is a free, open-source XML UI Language (XUL) toolkit in released under the GNU General Public License (GPL) that lets you build UIs using XML and includes an ultra-light weight, multi-threaded web server, a portal engine, a template engine (Velocity), a scripting interpreter (Python) and more.

SwingML
SwingML is a specification based on XML that releases the power of the JFC/Swing libraries making them easier to implement in the client side and providing an alternative to replace completely the use of HTML.

SwiXml
SwiXml, is a small GUI generating engine for Java applications and applets. Graphical User Interfaces are described in XML documents that are parsed at runtime and rendered into javax.swing objects.

Thinlet
Thinlet is a GUI toolkit, a single Java class, parses the hierarchy and properties of the GUI, handles user interaction, and calls business logic. Separates the graphic presentation (described in an XML file) and the application methods (written as Java code).

UIHierarchy
UIHierarchy is a library that facilitates creation of user interfaces. The main idea is to enforce the mental picture of the containment hierarchy of GUI components by writing the code in the same logical way. Code gets much easier to understand, much shorter, and easy to maintain. Additionaly, UIHierarchy can manipulate resulting components hierarchies in a powerful way.

WidgetServer
WidgetServer is a Java/XML server-side GUI-framework which enables an application to run as either a monolithic Swing app, a client/server Swing app, or as a Web app without any change. It includes a mixed deployment mode, which supports one server to serve Swing clients and Web clients at the same time.

XUI
XUI is a Java and XML framework for building rich client, desktop and mobile applications. The framework can save you up to 60% of the code typically needed to build an application. The result is real savings in development time and maintenance costs and greater stability.