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.
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.
TaskBoard - A Kanban-inspired app for keeping track of things that need to get done.
Pivotal Tracker - Pivotal Tracker: The awesome, lightweight, agile project management tool for software teams. Get your 30-day Free Trial started today!
Pintask - Pintask is the most customizable Kanban board.
Redmine - Flexible project management web application