iA Writer is a popular plain-text, Markdown writing app. Perhaps its most salient feature is its simplicity, which it achieves without sacrificing quality. I may be making you fine readers read too many writing and productivity app reviews, so for this one, I’ll just send you the relevant links so you can check out more.
Here‘s iA’s write-up of the app, with lots of details and links. Here it is in the Mac Store, and here it is in the iOS App Store. (They even have an Android version!)