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.
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.