Validation

Comparative Analysis of Six XML Schema Languages
Paper by Dongwon Lee and Wesley W. Chu comparing XML DTD, XML Schema, XDR, SOX, Schematron, and DSD.
http://www.cobase.cs.ucla.edu/tech-docs/dongwon/ucla-200008.html

Examplotron
Uses instance documents as a lightweight schema language; sample documents are marked up with the information needed to guide a validator.
http://examplotron.org/

Schemachine
Specification of a framework for modular validation of XML documents. [PDF]
http://www.topologi.com/public/Schemachine.pdf

Systemwire xlinkit
integrity management of distributed XML data. Provides a validation rules language and processing engine that allows constraints beyond schema and DTD validation.
http://www.systemwire.com/solutions/technology_overview.html

Trang
A open source multi-format schema converter based on RELAX NG. Includes documentation and download.
http://www.thaiopensource.com/relaxng/trang.html

XML Well-Formedness Checker and Validator
Checks an XML document (specified by URL) for well-formedness and optionally DTD-based validity and namespace correctness. Outputs canonical XML. Runs over the web and can be downloaded.
http://www.cogsci.ed.ac.uk/~richard/xml-check.html

XMLProbe
Automated data quality checking of XML content.
http://www.xmlprobe.com/