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.

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

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.

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.

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

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