Adobe XD - Adobe XD is an all-in-one UX/UI solution for designing websites, mobile apps and more.
Quip - Quip is the modern productivity suite that simplifies your life and helps your team get work done faster.
Figma - Team-based interface design, Figma lets you collaborate on designs in real time.
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.
Sketch + InVision - Design prototyping, collaboration & workflow for Sketch
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.