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.
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.
Basecamp - A simple and elegant project management system.
Remember The Milk - Remember The Milk is a task and time management application for mobile devices.
Smartsheet - Smartsheet is an intuitive online project management tool enabling teams to increase productivity using cloud, collaboration, & mobile technologies.
monday.com - The most intuitive platform to manage projects and teamwork