Quip - Quip is the modern productivity suite that simplifies your life and helps your team get work done faster.
Beyond Compare - Beyond Compare allows you to compare files and folders.
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.
WinMerge - WinMerge is an open source differencing and merging tool for Windows.
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.
Meld - What is Meld? Meld is a visual diff and merge tool targeted at developers.