Besides "The Big Three" dozens of Xul motors blossom or fade away.

• Peter Köker's GUI Builder builds Swing apps from a Xul dialect dubbed "GUI Design Language" (GDL) and supports scripting in Pnuts (http://javacenter.sun.co.jp/pnuts/). GUI Builder also includes a JavaDoc-like tool dubbed GuiDoc that automatically creates HTML documentation from Xul files (source available under the GNU Lesser Public License) - (Note: German site ahead) - http://www.guibuilder.de.
• Jim Wissner's Xybrix builds Swing apps dressed up in Kunststoff L&F from a Xul dialect that includes XForms support (source available under an MIT License) - http://www.jbrix.org/kits/xybrix/overview.html, http://sourceforge.net/projects/xybrix.
• Thinlets - a Xul motor packed into a 24 k jar runs Java 1.1 applet oldies - http://www.thinlet.com
• Pierre Morel's Proto, a Xul forerunner, builds Swing apps from XML descriptions and supports scripting in Javascript or Tcl (frozen since 1999) - http://www.pierlou.com/prototype/.