HackMD.io is an open source, collaborative Markdown editor. It allows people to share, comment, and collaborate on documents. As open source software, users can choose between using the online platform or installing it as a local service using the upstream project CodiMD.
HackMD's primary feature is obviously the text editor; it leverages the Markdown language, provides handy tools like inserting checkboxes and horizontal separator lines, and allows users to visualize the Markdown rendering while they're working on a document. But HackMD's real power is in enabling collaboration.
Etherpad and HackMD may both do online collaborative document editing but whilst Etherpad may be a simpler WYSIWYG editor, HackMD's claim to fame is its export to various HTML formats for easy use in websites and other HTML based documents and it can render better on mobile devices. And don't forget Cryptpad which focusses more on security for online collaboration. The notetaker I use that syncs with my NextCloud hosting (QownNotes) is a markdown editor too.
See https://opensource.com/article/19/7/enable-collaboration-hackmd Use HackMD to collaborate on open source projects
HackMD is a Markdown editor that also allows people to share, comment, and collaborate on documents.