Qt User Interface
Qt - http://www.trolltech.com/products/qt/index.html - is a comprehensive C++ application development framework. It includes a class library and tools for cross-platform development and internationalization.
The Qt User Interface (QTUI) is a K-3D user interface plugin that uses Qt to provide a simple-but-workable K-3D "reader" application that displays (but cannot modify) a K-3D document. It is not meant to replace or compete with the Next Generation User Interface, rather, it is a working demo of what can be done with K-3D user interface plugins.