Writer is an easy screenwriting application based on the fountain language (see http://www.fountain.io), which is similar to markdown but optimized for screenwriting by automatically recognizing e.g. scene headers or dialogue. Writer is a standard cocoa document based application for OS X.
The app can be downloaded from the Releases Page: https://github.com/HendrikNoeller/Writer/releases/tag/2.1.1
By Hendrik Noeller