Quip - Quip is the modern productivity suite that simplifies your life and helps your team get work done faster.
Typora - A minimal Markdown reading & writing app.
Google Docs - Create a new document and edit with others at the same time -- from your computer, phone or tablet. Get stuff done with or without an internet connection. Use Docs to edit Word files. Free from Google.
Dillinger - joemccann has 95 repositories available. Follow their code on GitHub.
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
StackEdit - Full-featured, open-source Markdown editor based on PageDown, the Markdown library used by Stack Overflow and the other Stack Exchange sites.