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.
Walling for Windows - Visual walls for your ideas and projects. Now on Windows.
GoodGantt - The best Gantt chart for Trello®
Smart Notes - Sticky notes for your web browser
Paws for Trello - The best way to use Trello on your desktop
Allo.io - Whiteboarding, documentation, and video chat in one place.