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.
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
KanbanFlow - KanbanFlow is a Lean project management tool allowing real-time collaboration between team members. Supports the Pomodoro technique for time tracking.
Meistertask - MeisterTask is an intuitive task management and collaboration tool.
Remember The Milk - Remember The Milk is a task and time management application for mobile devices.
Wekan - The open-source Trello-like kanban (built with Meteor)