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.
KanbanFlow - KanbanFlow is a Lean project management tool allowing real-time collaboration between team members. Supports the Pomodoro technique for time tracking.
MindMeister - Create, share and collaboratively work on mind maps with MindMeister, the leading online mind mapping software. Includes apps for iPhone, iPad and Android.
Meistertask - MeisterTask is an intuitive task management and collaboration tool.
Coggle - Coggle is a simple, beautiful, powerful way of structuring information.