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.
Agile Tools for Trello - The one power-up all agile teams using Trello need
GoodGantt - The best Gantt chart for Trello®
Trello for Slack - Organize anything with anyone, inside Slack
Paws for Trello - The best way to use Trello on your desktop
Rambox - Digital workspace organizer that allows you to unify as many applications as you want, all in one place. It is perfect for those who care about productivity while working with many business and personal apps.