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.
Strikethru - Strikethru is a kind of to-do list notebook for managing work lists and staying updated with the important tasks.