From Clemens Uhlenhut:
XML ValidatorBuddy is an easy to use XML/JSON editor and XML validator together with Schematron and batch validation. XML ValidatorBuddy makes working with XML files as easy as never before. The installation includes the Apache Xerces XML parser. Use the XML ValidatorBuddy desktop application as your starting point to create, edit, inspect, validate, convert and transform your XML and JSON files. Because XML ValidatorBuddy doesn't need to load the complete XML file into memory it is also possible to validate and view huge (multiple GB) instance documents. Use XML ValidatorBuddy to sign and verify one or multiple XML documents at once using digital signatures.
Take advantage of the grid-style JSON editor to always create error free JSON text.
The tool can also convert between XML, JSON and CSV formats. Convert CSV files to XML or JSON, between XML and JSON and from JSON to CSV. Several options are available for all types of conversions.
XML ValidatorBuddy supports all widely used XML and JSON technologies like W3C schema validation, DTDs, XPath evaluation, XSL and XSLT, Schematron, JSON Schema and JSON pointer.