The XML User Interface Language (XUL) is a markup language for describing user interfaces. With XUL you can create rich, sophisticated cross-platform web applications easily. Mozilla Firefox's user interface is mostly developped in XUL. XUL has encountered a high success recently thanks to this browser with the possibility to create Extensions. Here are some of my favorites:

• Web Developer Extension: Adds a menu and a toolbar with various web developer tools.
• Firebug: JavaScript debugger, DOM tree explorer, spy on XMLHttpRequest traffic, and many other features...
• Piggy-bank: turns Firefox into a "Semantic Web browser", letting you make use of existing information on the Web in more useful and flexible ways not offered by the original Web sites.

I started developing a new extension for my personal needs and found Xulplanet.com, which is THE resource of the web for developing in XUL. Xulfr proved to be very useful to me since their team translated the main XULplanet tutorial in French. (Some articles that have been removed from XULPlanet are still there !!) The combo Ajax/XUL/Rdf is really powerful for developping web applications manipulating many informations from many sources. Have fun !