Trix - A rich text editor for everyday writing.
Markdown by DaringFireball - Text-to-HTML conversion tool/syntax for web writers, by John Gruber
BBEdit - BBEdit is a professional HTML and text editor for Macintosh that provides features for editing, searching, and minipulation of text for web authors and software developers.
Typora - A minimal Markdown reading & writing app.
Draft.js - Rich Text Editor Framework for React
StackEdit - Full-featured, open-source Markdown editor based on PageDown, the Markdown library used by Stack Overflow and the other Stack Exchange sites.