Texts is a word processor that uses text files with markdown formatting for file storage. The extension is "text". However, there is nothing in the user interface that betrays it is a markdown editor. You use Ctrl-sequences for formatting, etc.
They call the application a Markdown Visual Editor: "Write using Markdown — without having to remember the markup. With Texts you can apply styles to words and paragraphs and immediately see the results. Your images and tables are displayed directly within Texts."
What I like especially is that you can easily add footnotes. They translate correctly into rtf and docx.
It's available for Windows and for both OSX and iOS. If I understand the Website correctly you get both the Windows and the OSX version when you pay. Right now, or "for a limited time," it's available for $14.50, that is, for a 50% discount. The iOS version is available through ITunes in Apps store for free.
Oh ... and did I mention that you have to download Pandoc for conversion to other formats? It's nowhere mentioned on the Website that I could see.
I like it a lot (after playing with it for a few hours—and even using it for serious work). It's a slick application. I am buying it ... in spite of David Hewson's criticism of Markdown for general writing. In any case, this application seems to escape his most fundamental criticism.
Again, I agree with Hewson that "anything that claims to unlock your creativity is probably snake oil ... ask yourself. How could this possibly work?"