iA Writer's Markdown support is very good. The app offers a formatting bar at the bottom of the window that lets you quickly apply Markdown formatting, even if you don't remember which syntax characters to use. You also get a decent Markdown-preview window, though it's not customizable.
In fact, that's one of iA Writer's hallmarks, love it or hate it: Little in the app is customizable. If you don't like what's here, you should look elsewhere. But if you like iA Writer's approach and attractive styling, you'll find it to be a great tool to write without distractions.
There are also iOS version of iA Writer, for iPad and iPhone, and those apps work with iCloud, but you can also use Dropbox to be able to access your files from any app.
Hog Bay Software's $10 WriteRoom is a bit like iA Writer with more options. WriteRoom comes with a handful of default themes, and you can download others or even roll your own, if you're so inclined. You can zoom your text and use any font installed on your Mac, and the app's Typewriter Mode automatically scrolls your document as you type, similar to how you'd see a piece of paper move in a typewriter. WriteRoom has no Markdown (or other language) preview, however, limiting its usefulness for writing in anything but plain text without an additional app to show what your writing will look like.
Metaclassy's $10 Byword falls, feature-wise, somewhere between iA Writer and WriteRoom — it lets you choose between a dark them and a light theme, and it lets you choose your font. But Byword gets Focus Mode right, allowing you to choose either Line Focus or Paragraph Focus. It's also got a typewriter mode, it supports rich text, and the premium version (unlocked via a $5 in-app purchase) lets you publish your writings directly to WordPress, Tumblr, Blogger, Scriptogr.am and Evernote.
Byword is also available in iPad and iPhone versions, and it syncs your files using iCloud. However, fans of Markdown will want to steer clear (unless they use Marked, below), as Byword's Markdown preview requires that you toggle between edit and preview mode — you can't proofread using the preview and make changes to your text at the same time.
Hog Bay Software offers an alternative to WriteRoom called FoldingText, which, while not exactly a focused-writing app, does something interesting. The $25 app is a combination of a text editor and an outliner: You format your text with Markdown syntax, but you can "fold," or hide, sections by clicking on header characters. This feature lets you write longer documents, keeping a big-picture view without having to tediously scroll up and down. You can also move sections around like an outliner, but FoldingText is still, at heart, an app for writing and working with text. When you've finished writing, you can export your document to HTML, or copy it to the clipboard as HTML or rich text.
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