Nullboard - Minimalist locally-stored kanban board / task list manager.
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.
Waffle - Split screen picture conversations with friends
GoodGantt - The best Gantt chart for Trello®
Brisqi - Offline-first personal Kanban app
Paws for Trello - The best way to use Trello on your desktop