HelpNDoc - What is HelpNDoc ? Easy to use yet powerful help authoring environment which can generate various documentation formats from a single source.
Altova XMLSpy - The XMLSpy XML Editor is a powerful tool for editing XML and related technologies. It is the only XML Editor with patented SmartFix validation, enterprise-grade editors, converters, debuggers, and code generators.
Dr.Explain - Dr.Explain software is a help file authoring tool.
XML Copy Editor - XML Copy Editor is a fast, free, validating XML editor.
MadCap Flare - Documentation for Any Audience, Language or Format
Liquid Studio - XML Tools Provider: XML Studio - Graphical XML Schema Editor.