mdbook - Gitbook alternative in Rust
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.
MkDocs - Project documentation with Markdown.
Notion - All-in-one workspace. One tool for your whole team. Write, plan, and get organized.
GitBook - Modern Publishing, Simply taking your books from ideas to finished, polished books.
Slack - A messaging app for teams who see through the Earth!