Liquid XML Studio 2012 is an advanced XML toolkit and IDE, containing all the tools needed for designing and developing XML applications, including an XML Editor, XML Schema Editor, WSDL Editor, Web Service Tools and much more.

Popular XML Editor and IDE including XML Schema designer, code generator, file converters, debuggers, profilers, support for XSLT, XQuery, WSDL, SOAP, plus .NET and Eclipse integration. [Commercial]

XML editor and XSLT debugger for Windows, Linux and Mac OS X. Provides editing support for XML, DTD, XHTML, XSLT, XSD, XML RelaxNG, SVG, MathML and XML FO [Commercial]

Sonic Software's platform for an XML development environment that includes an XML Schema editor, XSLT editor, XML-XML mapper, debugging and document wizards. [Commercial]

Offers a tree-table-text interface eith both hierarchical and tabular style views [Freeware]

A free Java based XML editor that can perform validation, XSL transformations, schema/DTD generation, code completion, and formatting. Details of features, project information, screen-shots, and download. [GPL]

XML Editor: Schema based validation & intellisense, Outlining & Auto Complete. The toolkit also include a graphical XML Schema (XSD) editor, XPath viewer, XML Diff tool, Web Service call composer, and more. [Freeware, Commercial]