Orbeon Studio
Oxygen http://www.oxygenxml.com/
JAPISoft's http://www.japisoft.com/exslt/
XMLAuthor http://www.svcdelivery.com/xmlauthor/
XMLBuddy http://www.xmlbuddy.com/ (Pro and free versions)
Compound XML Document Editor http://www.alphaworks.ibm.com/tech/cxde