Typora - A minimal Markdown reading & writing app.
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.
Notion - All-in-one workspace. One tool for your whole team. Write, plan, and get organized.
MarkText.app - Mark Text is a Markdown editor for Mac, Windows and Linux.
Slack - A messaging app for teams who see through the Earth!