QXmlEdit is a simple XML editor written in qt4. Its main features are unusual data visualization modes, it has some nice XML manipulation and presentation features and it is multi platform. This is one of the few graphical Open Source XSD viewer.
Latest release is 0.8.1. This version splits the XML editor into a separate library that can be embedded in other applications. Main new features are session management and suggestions from XSD schema, if available, when inserting elments, and graphical XML file view.