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.
Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.
Task Coach - Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists.
Taskade - Make lists, organize your thoughts, and be inspired to get things done. Taskade is a collaborative space for your tasks.
GTDNext - About Us.
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.