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