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A web-based XML validator. Can be used for free. Validates XML files, prompts the user for referenced files (e. g., XSD), shows good error messages.

W3C

XML.com, where the XML community shares XML development resources and solutions, features timely news, opinions, features, and tutorials; the Annotated XML specification created by Tim Bray; authoring tools, XML developer resources, interactive forum...

A fast and easy source-level XML Editor, XML Marker uses a combination of syntax-highlighting editor, tree view, and table views to help you get a better insight to your XML code

Free cross-platform web-based Online XML Editor, runs on Linux, Windows, Mac, FireFox, Chrome, IE. Edit XML schema or document. No user registration required.

Online tools for developers, including formatters (JSON, XML, HTML, SQL, etc.), minifiers (JavaScript, CSS), validators, compactors and many more!

oXygen XML Editor is a cross platform XML Editor providing tools for XML authoring, XML conversion, XSL, XSLT, XQuery, XML Schema, DTD, Relax NG and Schematron development, SOAP and WSDL.