Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.
Org mode - Org: an Emacs Mode for Notes, Planning, and Authoring
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.
QOwnNotes - QOwnNotes is a free plain-text notepad with markdown support for Linux, Mac OS X and Windows, that works together with ownCloud Notes.
Remember The Milk - Remember The Milk is a task and time management application for mobile devices.