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.
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.
Wekan - The open-source Trello-like kanban (built with Meteor)
Zenkit - Zenkit is the home you always wanted for your ideas, your projects, your business.
Asana - Asana project management is an effort to re-imagine how we work together, through modern productivity software. Fast and versatile, Asana helps individuals and groups get more done.