Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Getting Things GNOME - Getting Things Gnome! is an organizer for the GNOME desktop environment.
Remember The Milk - Remember The Milk is a task and time management application for mobile devices.
WeekToDo - Free and Open Source Minimalist Weekly Planner and To-Do list App Focused on Privacy
Things - Things is an easy to use task manager.
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.