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.
GoodGantt - The best Gantt chart for Trello®
SendBoard for Trello - Receive and reply to emails in Trello
Paws for Trello - The best way to use Trello on your desktop
Agile Tools for Trello - The one power-up all agile teams using Trello need
Teamgantt - Intuitive project planning and collaboration. Beautiful Gantt charts, task-level communication, file sharing, resource management and more.