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.
Notion - All-in-one workspace. One tool for your whole team. Write, plan, and get organized.
XML Copy Editor - XML Copy Editor is a fast, free, validating XML editor.
Google Docs - Create a new document and edit with others at the same time -- from your computer, phone or tablet. Get stuff done with or without an internet connection. Use Docs to edit Word files. Free from Google.
Oxygen XML Editor - Oxygen XML Editor provides a comprehensive suite of XML authoring and development tools.
Nuclino - Nuclino works like a collective brain, helping teams bring all their knowledge, docs, and projects together in one place. It's a modern, simple, and blazingly fast way to collaborate.