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.
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
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.
hiTask - HiTask is a free shared task list for your team. The place to plan, organize, and stay in sync.
Basecamp - A simple and elegant project management system.
Zenkit - Zenkit is the home you always wanted for your ideas, your projects, your business.