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.

Task Coach - Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists.

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

Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.

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

EssentialPIM - EssentialPIM is a free Personal Information Manager that keeps up with the times and lets you manage appointments, tasks, notes, contacts, password entries and email messages across multiple devices and cloud applications.