RELAX NG
DocBook RELAX Schema, RELAX Core (ISO/IEC TR 22250-1), RELAX NG specification, TREX - Tree Regular Expressions for XML
Schematron
Schematron reference at Zvon, Schematron Specification, Schematron Tutorial at Zvon, Topologi Schematron Validator
XML Schema
FAQs, Help and Tutorials, Standards Documents, Tools, W3C Official Public Site for XML Schema, W3C XML Schema Test Collection, XML Schema Reference at Zvon
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.
DCD - Document Content Description for XML
This document proposes a structural schema facility, Document Content Description (DCD), for specifying rules covering the structure and content of XML documents. [W3C Note 31 July 1998]
DSD - Document Structure Description
An XML schema language. A DSD document is a specification of a class of XML documents together with a default mechanism and documentation. FAQs, downloads, tutorials provided.
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.
SOX - Schema for Object-Oriented XML 2.0
A schema language for defining the syntactic structure and partial semantics of XML document types; an alternative to XML DTDs. [W3C Note 30 July 1999]
Systemwire xlinkit
integrity management of distributed XML data. Provides a validation rules language and processing engine that allows constraints beyond schema and DTD validation.
XDR - XML-Data / XML-Data Reduced
Describes an XML vocabulary for schemas, that is, for defining and documenting object classes. [W3C Note 05 Jan 1998]
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.