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.

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

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

QOwnNotes - QOwnNotes is a free plain-text notepad with markdown support for Linux, Mac OS X and Windows, that works together with ownCloud Notes.