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.

hiTask - HiTask is a free shared task list for your team. The place to plan, organize, and stay in sync.

Todoist - Todoist is a to-do list that helps you get organized, at work and in life.

Mail Pilot - Beautiful task oriented email client for Mac, iPhone, & iPad

Ora - Welcome to your team’s command center. A place to track projects. Tasks. Time. Commits. Status Reports. Your entire product or business! It's the fastest-growing productivity suite your team needs right now!

Toggl - Toggl is an online time tracking tool. It features 1-click time tracking and helps you see where your time goes. Free and paid versions are available.