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.
Elegantt for Trello - The most elegant way to manage your Trello projects
Agile Tools for Trello - The one power-up all agile teams using Trello need
Hello Epics for Trello - Simplify your Trello board with parent/child relationships
Trello for Slack - Organize anything with anyone, inside Slack
TunnelVision - See your next Trello tasks every time you open a browser tab