Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Joplin - Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, tagged and modified either from the applications directly or from your own text editor.
Org mode - Org: an Emacs Mode for Notes, Planning, and Authoring
CherryTree - A hierarchical note taking application, featuring rich text and syntax highlighting, storing data in a single xml or sqlite file.
Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.
OneNote - Get the OneNote app for free on your tablet, phone, and computer, so you can capture your ideas and to-do lists in one place wherever you are. Or try OneNote with Office for free.