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.
Hello Epics for Trello - Simplify your Trello board with parent/child relationships
GoodGantt - The best Gantt chart for Trello®
TunnelVision - See your next Trello tasks every time you open a browser tab
Airtable - Airtable works like a spreadsheet but gives you the power of a database to organize anything. Sign up for free.
Teamgantt - Intuitive project planning and collaboration. Beautiful Gantt charts, task-level communication, file sharing, resource management and more.