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.

Streaks - The to-do list that helps you form good habits.

TaskBoard - A Kanban-inspired app for keeping track of things that need to get done.

Taskful - Deadlines, meet your match.

LeanKit by Planview - See how Planview LeanKit enables engineering teams across all levels of the organization with a visual work delivery tool to apply Lean management principles to their work, helping them work smarter and deliver faster.

ToDoList - Windows desktop application to stay on top of your daily tasks.