My pal Brett Terpstra recently released Marked, version 2.5. If you're not familiar with Marked, it's a native macOS app for previewing Markdown and other text markup languages, as well as HTML and OPML files. With Marked, you can see rendered, styled versions before publishing. Marked 2 updates with changes whenever you save using the text editor of your choice. In combination with your favorite Markdown editor (or any text editor), Marked 2 is an excellent writing tool, particularly if you write in Markdown.
There's a bunch of new features with the new version including support for a pile of new applications (including Scrivener 3). Spelling and grammar check is also now included for all users. (It no longer requires an in-app purchase.)