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.
Plan.io - Planio makes web based project management and team collaboration more efficient and fun. It is the perfect platform for your projects, team members and clients.
Meistertask - MeisterTask is an intuitive task management and collaboration tool.
TargetProcess - Agile Project Management Web Application