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

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

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

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

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/

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

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