It's a new markdown app for Windows, Mac and Linux. It's in beta, but looks promising.
Ranked in these QuestionsQuestion Ranking
Pro Free and open source
Pro Readable single-pane inline preview editor
Inline preview is well formatted and easy to read, making it a good choice for single-pane editing.
Pro Table of contents is populated with links
The table of contents pane has links in it, which makes clicking through to the section you want really easy.
Con No package
Must be run from the file directly or built from source.