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.
Google Calendar - Spend less time managing your day & more time enjoying it
GoodGantt - The best Gantt chart for Trello®
Mixmax Calendar - The fastest way to schedule meetings with anyone, anywhere
Paws for Trello - The best way to use Trello on your desktop
Sunsama - Calendar and scheduling for teams