WEBsina
Java application software for license key generation, bug tracking, defect tracking, and issue tracking.
http://www.websina.com/products/

Lark and Larval
Lark is a non-validating processor implemented in the Java language; it attempts to achieve good trade-offs among compactness, completeness, and performance. Larval is a validating processor built on the same code base as Lark.
http://www.textuality.com/Lark/

RXP
A validating parser written in C and available under the GPL.
http://www.cogsci.ed.ac.uk/~richard/rxp.html

XML Toybox
Xmlu.com: Enabling a New Generation of Web Developers. Xmlu.com is your single source for information and training about XML. Xmlu.com provides a business-oriented view of XML.
http://architag.com/xmlu/play

Expat
A non-validating XML 1.0 parser written in C.
http://www.jclark.com/xml/expat.html

XP
Non-validating XML parser. [Open Source, BSD-like]
http://www.jclark.com/xml/xp/index.html

XED
Text-based non-validating XML editor for Windows, UNIX.
http://www.ltg.ed.ac.uk/~ht/xed.html

Xparse
Non-validating XML parser running in the client browser.
http://www.jeremie.com/Dev/XML/

XML Zoo
Interactive XML tutorials. Students can practice validating and processing using XSD Schema, XSLT, DTD, DOM, SAX and SVG.
http://xmlzoo.net

Xparse
Non-validating library written in Javascript, runs in the client browser.
http://www.jeremie.com/Dev/XML/