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.
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.
XSLT in Perspective, slides for a talk on XSLT (collected into a single HTML page).
XML Namespaces, an explanation of the XML Namespaces Recommendation.