Mac: You have a lot of options for apps when it comes to writing Markdown style, but MarkDrop is an app that differentiates itself by making it dead-simple to also share the files you create. You can export your text in a variety of formats, change the look and theme, and write in dark screen mode.
What makes MarkDrop interesting is the fact that it integrates with Droplr. So, all the files you create are instantly saved online and can be read in a browser by anyone you share the link with. It's a simple way to share a lot of words with people without dealing with the usual upload hassles.
MarkDrop ($4.49 today only) [Mac App Store]