CKEditor - Real-time collaborative future-ready rich text 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.
TinyMCE - TinyMCE is a content editor that functions as a plug-in for Wordpress websites.
Boostnote - Boostnote is an open-source note-taking app.
Aloha Editor - Aloha Editor is an HTML5 WYSIWYG Editor that gives you a total new experience when editing.
Standard Notes - A safe place for your notes, thoughts, and life's work