A guided tutorial that will help you get started with XUL, originally from XULPlanet.
XUL (XML User Interface Language) is Mozilla's XML-based language for building user interfaces. In particular, it is used by Firefox. Web developers already familiar with HTML will learn XUL quickly and can start building applications right away. Open XUL Periodic Table in Firefox or another Gecko-based browser to see some XUL demos.
Note: This "XUL Periodic Table" cannot be shown in recent versions of Gecko because it uses remote XUL, a technology whose support was dropped because of potential security issues. There is a XULRunner application containing the XUL Periodic Table which can be opened with Gecko based browsers. You can get it here. See: XULRunner_tips#Using_Firefox_3_to_run_XULRunner_applications for instructions on running XULRunner apps in Firefox.