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

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

Typora - A minimal Markdown reading & writing app.

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.

MarkText.app - Mark Text is a Markdown editor for Mac, Windows and Linux.

SiYuan - SiYuan is a desktop Markdown Block-Reference and Bidirectional-Link note-taking application. Support fine-grained block-level reference and Markdown WYSIWYG.