MacDown - MacDown is an open source Markdown editor for OS X, released under the MIT License.
SiYuan - SiYuan is a desktop Markdown Block-Reference and Bidirectional-Link note-taking application. Support fine-grained block-level reference and Markdown WYSIWYG.
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.
Markdown by DaringFireball - Text-to-HTML conversion tool/syntax for web writers, by John Gruber
Typora - A minimal Markdown reading & writing app.