Docusaurus - Easy to maintain open source documentation websites
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.
DocFX - A documentation generation tool for API reference and Markdown files!
Notion - All-in-one workspace. One tool for your whole team. Write, plan, and get organized.
Doxygen - Generate documentation from source code
Slack - A messaging app for teams who see through the Earth!