From the folks behind Bear:
Lettera is a native, refined Markdown editor for macOS. Built for writers, researchers, developers, and anyone who works with documents, from a quick blog draft to a complete technical documentation system. Lettera works around your setup. Open a single file to read and edit it, or open a folder as your writing workspace.
I dig it. See also: Vergecast interview with Chairman Gruber on the epic history of Markdown.