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.
Wrike - Wrike is a flexible, scalable, and easy-to-use collaborative work management software that helps high-performance teams organize and accomplish their work. Try it now.
Remember The Milk - Remember The Milk is a task and time management application for mobile devices.
Basecamp - A simple and elegant project management system.
WorkFlowy - WorkFlowy is an organizational tool that makes life easier. It's a surprisingly powerful way to take notes, make lists, collaborate, brainstorm, plan and generally organize your brain.
Smartsheet - Smartsheet is an intuitive online project management tool enabling teams to increase productivity using cloud, collaboration, & mobile technologies.