The following information comes from a colleague on an XML interest distribution list here at Xerox. It has some very useful links.
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1. Think of this as lint for your XML Schema:
http://www.alphaworks.ibm.com/tech/xmlsqc
Reads Schemas XML Schemas and attempts to determine if they are 100% valid under all the various constraints that apply to schemas.
If it determines they are not valid, it attempts to explain the problem in language that a schema novice could probably understand.
2. Good, long XML Schema tutorial with an example for every problem:
It's a ZIP file with 3 PPT presentations and examples. Each page of the PPT addresses a specific scenario.
http://www.xfront.com/xml-schema.html
3. A shorter XML Schema tutorial with examples:
http://www.zvon.org/xxl/XMLSchemaTutorial/Output/series.html
4. Command-line validator in Python (single-click install in Windows as Linux)
Download from: http://www.ltg.ed.ac.uk/~ht/xsv-status.html
5. An online HTML form-based interface for validating schemas and instance documents using.NET framework.
Use online: http://www.gotdotnet.com/services/xsdvalidator/
Download at: (broken link; reporting to Microsoft).
6. Online Schema Inference
Creates an XML Schema definition language (XSD) schema from an XML instance document.
When provided with well-formed XML file, the utility generates an XSD that can be used to validate that XML file.
You can also refine the XSD generated by providing the tool more wellformed XML files.
Use online: http://www.gotdotnet.com/team/xmltools/xsdinference/
Download at: http://www.gotdotnet.com/team/xmltools/xsdinference/XSDInference.exe