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.

Taiga - Project management platform for agile developers & designers

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.

Kanbanchi - Based on kanban, a scheduling system that originated in Japan, Kanbanchi lets teams collaborate in a drag-and-drop environment that enables reviewing multiple projects' highlights on one screen... read more.

Wekan - The open-source Trello-like kanban (built with Meteor)