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.
Beyond Compare - Beyond Compare allows you to compare files and folders.
Notion - All-in-one workspace. One tool for your whole team. Write, plan, and get organized.
WinMerge - WinMerge is an open source differencing and merging tool for Windows.
Slack - A messaging app for teams who see through the Earth!
Meld - What is Meld? Meld is a visual diff and merge tool targeted at developers.