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.

Pintask - Pintask is the most customizable Kanban board.

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.

Basecamp - A simple and elegant project management system.

buckets.co - Feature rich kanban board collaboration platform.