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.
Active Collab - Active Collab — Task management, Team collaboration, Time tracking & Invoicing app
GoodGantt - The best Gantt chart for Trello®
Agantty - Free, beautiful and easy to use project management tool
Paws for Trello - The best way to use Trello on your desktop
Gantt - Create beautiful Gantt charts