XML Copy Editor - XML Copy Editor is a fast, free, validating XML editor.
Outspline - Extensible outliner and personal time organizer to manage todo lists, schedule tasks, remind events.
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.
EditiX - EditiX is a quality Open Source XML Editor and XSLT Editor for Windows, Mac OS X and Linux. Test for Free with our 30-day Trial !
Online XML Editor - Guide to learn programming
Dynalist - Dynalist is a web app that lets you break down and organize your thoughts in the format of lists.