Fast and easy to use Agile project manager with great oficial iOS app both for iPhone and iPad HD versions and some good, but not perfect, no oficial Android apps

Pivotal Tracker is the best agile project management tool I have used.  Very easy to collaborate with others as you can attach files, make notes, assign tasks, delegate importance/duration, and everything can be seen in one view. It integrates with great products such as Zendesk and just allows such a simple view of everything going on, even on really complex projects.

Plus, it's obvioulsy a cloud product so I can log into Pivotal Tracker from anywhere and get up to speed and start completing tasks.