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.
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
Momentum - Agile goal and performance management
Trello for Slack - Organize anything with anyone, inside Slack
Hello Epics for Trello - Simplify your Trello board with parent/child relationships