TREX, a new, simple schema language for XML
The specification of a subset of XML called Canonical XML for use in testing XML parsers.
A collection of test cases for testing XML parsers. The valid test cases in the collection include the corresponding canonical XML.
A Java program XMLTest which generates canonical XML. This uses David Megginson's SAX API, and can be used with any parser which has a SAX driver.
XP, a high-performance XML parser in Java
Expat (XML Parser Toolkit), a library for XML parsing in C. This is the parser being used to add XML support to Netscape 5 and Perl. There are answers to some frequently asked questions about expat. There's also a useful article on expat at XML.com.
SP, which has enhanced XML support in version 1.3 both for parsing XML and for converting SGML to XML.
XT is an implementation in Java of XSL Transformations (XSLT).
XSLT in Perspective, slides for a talk on XSLT (collected into a single HTML page).
XML Namespaces, an explanation of the XML Namespaces Recommendation.