IBM's XML for Java parser (XML4J) is a high performance, modular XML
parser written in 100% Java.  XML4j makes it easy to give an application
the ability to read and write XML data.  A single JAR file provides classes
for parsing, generating, manipulating, and validating XML documents.
XML4J is the most robust XML processor currently available and is most
faithful to the XML 1.0 Recommendation and associated standards
(DOM 1.0, SAX 1.0, Namespaces).

WWW: http://www.alphaworks.ibm.com/tech/xml4j/