Gone - An ephemeral to-do list
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Streaks - The to-do list that helps you form good habits.
Getting Things GNOME - Getting Things Gnome! is an organizer for the GNOME desktop environment.
WeekToDo - Free and Open Source Minimalist Weekly Planner and To-Do list App Focused on Privacy
Taskade - Make lists, organize your thoughts, and be inspired to get things done. Taskade is a collaborative space for your tasks.