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.
Boardly - A minimalistic planning board for your projects
GoodGantt - The best Gantt chart for Trello®
Corrello - Dashboards for agile teams using Trello
Paws for Trello - The best way to use Trello on your desktop
Wiplo - The easiest way to organize and run your business workflows.