Turtl - The secure, collaborative notebook
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.
Tomboy - Apps/Tomboy - GNOME Wiki!
Notion - All-in-one workspace. One tool for your whole team. Write, plan, and get organized.
Carnet - Opensource rich text note taking app
Slack - A messaging app for teams who see through the Earth!