Luxor is an open-source XML User Interface Language (XUL) toolkit in Java that lets you build UIs using XML and also includes an ultra light-weight, multi-threaded Web server, a portal engine, and a template engine. It is also Web Start-ready, as everything fits in a jar and requires no loose files.
SwiXAT is a Swing-based authoring tool for the quick and easy development of GUI Java applications. It implements a true MVC framework and uses XML to define the view (SwiXML is used as the XUL engine), BeanShell as a scripting language for the controller, and JXPath as the binding mechanism between the view and model. It provides a complete environment in which it is possible to almost write an entire Java Swing application without writing Java compiled code. The use of XPath makes it very simple to traverse the object tree of the application's business model.
Google Gadgets for Linux provides a platform for running Google Gadgets under Linux, catering to the unique needs of Linux users. It's compatible with the gadgets written for Google Desktop for Windows as well as the Universal Gadgets on iGoogle. There are two main components to the application: one is a common gadget library responsible for running and presenting a gadget, and the other is a host program that allows the user to choose gadgets and run them on the desktop. Currently it has hosts written for GTK+ and Qt, with the GTK+ host offering a sidebar similiar to that of Google Desktop for Windows.
XWidglets is a complete rich Java Swing tool for designing, creating, and using XML-based GUIs. It provides a lightweight XML client, clear separation between view, data, and process, MVC I and II support, event management, a consistent look and feel across platforms, a rich set of GUI components, and n-tier architecture integration.
Apollo is an open-source developer test skeleton toolkit for Java Web Start/JNLP. It lets you turbo-charge Web Start apps without Web Start to speed up your compile/run/test/debug/goof-off cycle, avoiding the hassle of stuffing, signing, uploading, or downloading your jars every time you rearrange a comma in your source code.