On-Line XML Validator

An XML validator ensures that an XML data document is valid. The are many XML validators available on the web. An XML definition document(s) is used to validate a data file.

Here is a website that will validate XML Schema Documents (along with a brief tutorial):

How to use the XSD Schema Validator

XML Schema Validator

The Xerces XML-Validator is part of the Apache Project and is a great example of a working XML validator that can be integrated into software.

Apache XML Project / Xerces