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.
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
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.
Getting Things GNOME - Getting Things Gnome! is an organizer for the GNOME desktop environment.
Standard Notes - A safe place for your notes, thoughts, and life's work
WeekToDo - Free and Open Source Minimalist Weekly Planner and To-Do list App Focused on Privacy