XPath Visualizer:
http://www.vbxml.com/xpathvisualizer/default.asp

Apache FOP:
http://xml.apache.org/fop/index.html

DTDGenerator:
http://www.pault.com/pault/dtdgenerator/

Microsoft XML Validator:
http://msdn.microsoft.com/downloads/samples/internet/xml/xml_validator/

Java 2 Standard Edition:
http://java.sun.com/j2se

Java API for XML Processing (JAXP):
http://java.sun.com/xml/download.html

XT:
http://jclark.com/xml/xt.html

W3C:
http://www.w3.org

Apache XML project:
http://xml.apache.org

IBM XML DeveloperWorks:
http://www-106.ibm.com/developerworks/xml/

XML Spy:
http://www.xmlspy.com/

XML Software:
http://www.xmlsoftware.com/