XUL

XML User Interface Language

developed for Mozilla

http://www.mozilla.org/projects/xul/

http://xul.sourceforge.net/ Open Xul Alliance - links to various toolkits and activities

Lu Xor http://luxor-xul.sourceforge.net/ toolkit seems to focus on Java, with some Python use for scripting.

Thin Let generates Java applets for mobile devices (Cell Phone/PDA). Life Blogger uses it.