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.
TargetProcess - Agile Project Management Web Application
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.
Taiga - Project management platform for agile developers & designers
Pivotal Tracker - Pivotal Tracker: The awesome, lightweight, agile project management tool for software teams. Get your 30-day Free Trial started today!
Hygger - Hygger - is an Agile project management tool with built-in prioritization.