XmdTk is a Perl Tkx abstraction library that aims to provide cross platform, intuitive, object oriented approach to building graphical user interfaces from code and preconfigured XML files.
JAST (Javascript Toolkit) is an Open Source cross-platform javascript framework that aid users to perform the most hard operations with few simple lines of code.
It's modularity allow the download only for the code required to the page.
A lightweight cross platform XML editor. It has a tree viewer/editor and supports content completion, XPath, XQuery, XSLT, XML document validation etc. Many GUI features are configurable and supports multiple languages (localization).