Quip - Quip is the modern productivity suite that simplifies your life and helps your team get work done faster.
Crowdin - Localize your product in a seamless way
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.
POEditor - The translation and localization management platform that's easy to use *and* affordable!
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.
Lokalise - Localization tool for software developers. Web-based collaborative multi-platform editor, API/CLI, numerous plugins, iOS and Android SDK.