HeySpace - HeySpace is the only free task and simple project management software with communication and collaboration feature.
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®
Jeski - Many projects, multiple boards, one Jeski
Paws for Trello - The best way to use Trello on your desktop
Trello for Slack - Organize anything with anyone, inside Slack