Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.
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.
Task Coach - Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists.
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.
Strikethru - Strikethru is a kind of to-do list notebook for managing work lists and staying updated with the important tasks.
Standard Notes - A safe place for your notes, thoughts, and life's work