I've just bought Clean Writer Pro from the Mac App Store, to use it to write the articles in this site, I usually use vim, but I like testing new software, so I'm giving this one a try.
If you buy it, (It is for sale now only 99 cents) the first thing you will notice is that it is really minimalistic, if you put it in "full screen mode" you will get no distractions on the screen, just your text.
I also like the in-build spell checker, and grammar checker. One good feature is that it highlights the line you are working on, and fade out all the others, so you can focus on where you are in the document.
If you write markdown, you will love it, as it has a "Markdown Preview" feature, and you can see how your final text is going to look, but you can also copy the html output to paste it directly in your web server or in you blog. All of that with two clicks of the mouse, or one keyboard shortcut.
I really recommend it, if you like to write with no distractions, and focussed on your job.