Evernote - Bring your life's work together in one digital workspace. Evernote is the place to collect inspirational ideas, write meaningful words, and move your important projects forward.
XML Copy Editor - XML Copy Editor is a fast, free, validating XML editor.
Joplin - Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, tagged and modified either from the applications directly or from your own text editor.
Oxygen XML Editor - Oxygen XML Editor provides a comprehensive suite of XML authoring and development tools.
Google Keep - Capture notes, share them with others, and access them from your computer, phone or tablet. Free with a Google account.
Liquid Studio - XML Tools Provider: XML Studio - Graphical XML Schema Editor.