Editors

Altova XMLSpy XML Editor
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]
http://www.altova.com/products/xmlspy/xml_editor.html

EditiX
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.
http://www.editix.com

Liquid XML Editor
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]
http://www.liquid-technologies.com/XmlStudio/Free-Xml-Editor.aspx

Stylus Studio
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]
http://www.stylusstudio.com

XML Marker
Offers a tree-table-text interface eith both hierarchical and tabular style views.
http://symbolclick.com

XPontus
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]
http://xpontus.sourceforge.net/