MacDown is an open source Markdown editor for OS X, released under the MIT License. It is heavily influenced by Chen Luo’s Mou.
Here are a few features MacDown offers:
Hoedown is used internally to render Markdown into HTML. This makes MacDown’s live preview both efficient and very configurable. It also supports lots of non-standard syntactic features, including the very widely-used fenced code blocks with language identifiers. You can find all the available configurations in the Preferences.
MacDown offers syntax highlighting in fenced code blocks with language identifiers through Prism.
Auto-completion is implemented to MacDown to suit your needs.