QXmlEdit is a simple XML editor written in qt4. It has some nice XML manipulation and it is multi platform. This is one of the few graphical Open Source XSD viewer.
Main features: compact view of items, copy and paste of a whole subtree, fast expansion and compression of a hierarchy, compact edit of elements, base 64 data handling, snippets, styles, fonts zoom, columnar view, sessions handling, graphical XML file view, visual compare of XML Schema files.
