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®
Zoho Connect - Zoho Connect is a team collaboration software that helps teams ideate, discuss, and work together, from any place, on any device.
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
Airtable - Airtable works like a spreadsheet but gives you the power of a database to organize anything. Sign up for free.