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.

MarkNote - A simple markdown note management app.

Google Keep - Capture notes, share them with others, and access them from your computer, phone or tablet. Free with a Google account.

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

MarkFlowy - Modern markdown editor application, Build fast and efficient workflows.