Quip - Quip is the modern productivity suite that simplifies your life and helps your team get work done faster.
Zeplin - Collaboration app for UI designers & frontend developers
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.
Abstract - A secure, version-controlled hub for your design files
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.
Mightytext - Send & Receive SMS Text Messages from your computer. Sync'd with your Android #