MacDown - MacDown is an open source Markdown editor for OS X, released under the MIT License.
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.
Zettlr - Write Markdown documents with a comprehensive GUI and many workflow/time management tools.
Markdown by DaringFireball - Text-to-HTML conversion tool/syntax for web writers, by John Gruber
Notion - All-in-one workspace. One tool for your whole team. Write, plan, and get organized.
Typora - A minimal Markdown reading & writing app.