Probably some old news, but today I stumbled over the Microsoft XSD Inference tool:
The Microsoft XSD Inference utility is used to create an XML Schema definition language (XSD) schema from an XML instance document. When provided with wellformed 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. The download contains the core library, simple command line utility and the source code.
There's an online version available, but you can also download it for offline use. On this GotDotNet page are some more tools and articles listed: intresting stuff...
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