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.
Hillo - Board activity tracking for Trello
GoodGantt - The best Gantt chart for Trello®
Paws for Trello - The best way to use Trello on your desktop
Scrumpy - A simple, beautiful and agile project management software
Airtable - Airtable works like a spreadsheet but gives you the power of a database to organize anything. Sign up for free.