XML Copy Editor - XML Copy Editor is a fast, free, validating XML editor.
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.
Oxygen XML Editor - Oxygen XML Editor provides a comprehensive suite of XML authoring and development tools.
XMLBlueprint - XMLBlueprint is the smart and lightweight XML editor for Windows.
Online XML Editor - Guide to learn programming
TreeLine - TreeLine just stores almost any kind of information.