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.
Omni Notes - Note taking open-source application aimed to have both a simple interface but keeping smart behavior
Notion - All-in-one workspace. One tool for your whole team. Write, plan, and get organized.
Tomboy - Apps/Tomboy - GNOME Wiki!
Slack - A messaging app for teams who see through the Earth!