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®
Planyway - Visual project management for Jira and Trello: manage resources, build project roadmaps, use time tracking.
Paws for Trello - The best way to use Trello on your desktop
Corrello - Dashboards for agile teams using Trello
Airtable - Airtable works like a spreadsheet but gives you the power of a database to organize anything. Sign up for free.