WriteNext - The writing application that boosts your writing. Increase focus and writing performance and reduce distractions by separating your creative writing from other activities. Let writing be the only activity you perform in the writing application.
Quip - Quip is the modern productivity suite that simplifies your life and helps your team get work done faster.
Calmly Writer - Distraction-free text editor for Mac, Windows, Linux and ChromeOS.
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.
Stashany - Online notepad for developers
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.