This presentation: http://www.cafeconleche.org/slides/stpcon2005/testingxml/
XMLUnit: http://xmlunit.sourceforge.net/
XOM: http://www.xom.nu/
Schematron: http://www.schematron.com/resources.html
Canonical XML: http://www.w3.org/TR/xml-c14n
XML Infoset: http://www.w3.org/TR/xml-infoset