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