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.

MacDown - MacDown is an open source Markdown editor for OS X, released under the MIT License.

Zettlr - Write Markdown documents with a comprehensive GUI and many workflow/time management tools.

Notion - All-in-one workspace. One tool for your whole team. Write, plan, and get organized.

Obsidian.md - A second brain, for you, forever. Obsidian is a powerful knowledge base that works on top of a local folder of plain text Markdown files.

Notepad App - A markdown-based notes app for macOS, with simplicity and moderation.