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.
Xmind - Xmind is a brainstorming and mind mapping application.
Notion - All-in-one workspace. One tool for your whole team. Write, plan, and get organized.
MindManager - With MindManager, flexible mind maps promote freeform thinking and quick organization of ideas, so creativity and productivity can live in harmony.
Slack - A messaging app for teams who see through the Earth!
MindMeister - Create, share and collaboratively work on mind maps with MindMeister, the leading online mind mapping software. Includes apps for iPhone, iPad and Android.