Nifty - Manage projects, work, and communications in one place.
Typora - A minimal Markdown reading & writing app.
ZenHub - ZenHub is how the world's best software teams work together. Trusted by NASA, Docker, and Rackspace. Start free today!
Markdown by DaringFireball - Text-to-HTML conversion tool/syntax for web writers, by John Gruber
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.
Dillinger - joemccann has 95 repositories available. Follow their code on GitHub.