CKEditor - Real-time collaborative future-ready rich text editor
LibreOffice - Writer - Open-source cross-platform word processor.
Edtr.io - Intuitive WYSIWYG web editor, open source & fully adaptable.
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.
Summernote - Summernote is a JavaScript library that helps users create WYSIWYG editors online.
Microsoft Word - Microsoft Word is a commercial word document processor for Windows.