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.
Redactor Text Editor - WYSIWYG html editor.
Boostnote - Boostnote is an open-source note-taking app.
PHP Markdown - PHP port of the Markdown text-to-HTML conversion tool for web writers.
Standard Notes - A safe place for your notes, thoughts, and life's work