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

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.

TickTick - TickTickis a cross-platform to-do list app & task manager helps you to get all things done and make life well organized.

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

Remember The Milk - Remember The Milk is a task and time management application for mobile devices.

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