Xopus

Xopus is a WYSIWYG XML Editor that runs in your browser. Xopus enables end-users to work with structured and complex content without the need for technical knowledge. Xopus presents the user with a friendly interface to XML. Integrating Xopus requires knowledge of XML, XSD, XSL and some Javascript.

But you’d better let them tell you all about it:

www.xopus.com