See also:

• Computers: Data Formats: Markup Languages: HTML: Tools: Validators and Lints  (22)
• Computers: Data Formats: Markup Languages: XML: Applications  (145)
• Computers: Data Formats: Markup Languages: XML: Tools: Editors: Schema and DTD  (6)
• Computers: Data Formats: Markup Languages: XML: Tools: Parsers  (12)

• 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.
• Examplotron - Uses instance documents as a lightweight schema language; sample documents are marked up with the information needed to guide a validator.
• Schemachine - Specification of a framework for modular validation of XML documents. [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.
• Trang - A open source multi-format schema converter based on RELAX NG. Includes documentation and download.
• 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.
• XMLProbe - Automated data quality checking of XML content.