The rest of the pack
TextEdit Many people can get along just fine with OS X's built-in TextEdit, which is actually a full-blown word processor. It can handle serious formatting, such as lists and tables; can work with graphics, videos, and audio files; and can export to a variety of formats, including RTF and Word. But despite its many formatting options, TextEdit is a "plain text" editor at heart — it's the direct descendant of Apple's aptly-named SimpleText, and the much older TeachText, both simple tools designed for editing and displaying text documents. It also offers no true distraction-free mode, doesn't include full-screen mode, and doesn't explicitly support Markdown formatting, though if you use it in conjunction with Marked 2 (see below), you can write in Markdown and preview your formatted documents.
Mou Chen Luo's Mou is a free (donation requested), Markdown-focused text editor that includes a live preview in a two-pane window: editor on the left, preview on the right. The app includes keyboard shortcuts for Markdown-syntax formatting, and notably offers very good support for CJK (Chinese, Japanese, Korean) languages. It's also very flexible, letting you choose themes — including those you create — for both the composer and preview panes. Mou even lets you post directly to Tumblr and Scriptogr.am, though not to WordPress or Blogger.
However, placing the editor and preview pane in a single window takes up a lot of space, and if you've got a dual-display setup, you can't separate those views so that the editor resides on one display and the preview on the other. You can temporarily hide the preview pane, but the window doesn't resize, instead expanding the editor pane to twice its width, which can be annoying. Also, you can't choose which side of the editor the preview pane appears on — I prefer to have my preview to the left of my editor, and I can't do that with Mou.
For Markdown fans: Marked
While all of my recommended focused-writing apps handle Markdown syntax to some extent, few shine when it comes to previewing the formatted output (HTML or RTF, for example) of a Markdown document. iA Writer's preview isn't customizable; Byword requires you to toggle between composing and previewing; and WriteRoom has no Markdown preview at all.
I've adopted Brett Terpstra's $12 Marked (currently at version 2), which not only provides Markdown previews that are automatically updated to reflect changes you make to your documents, but also lets you customize its display with CSS files that, for example, match your blog or website's design. Marked also includes a full range of text-statistic tools, as well as export options — so even if your favorite writing app doesn't export to your preferred format, it's likely that you can get such functionality from Marked. In my writing, Marked has proven invaluable, especially for documents with complex formatting — not only is its export-to-HTML feature perfect, but I use my website's own CSS file for Marked's preview, so I can see exactly how my writing will look. Marked can also display inline images based on Markdown and HTML links, something the writing apps above can't do.
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