To-Do is free. Not a free tier, just free. If you’ve used or heard of Wunderlist (which was a popular task app), Microsoft bought them and created To-Do out of that expertise. There is the usual assortment of clients: web, iOS, Android, Windows and Mac. And the desktop apps don’t seem to be wrappers of the web app, they are nice native apps. If this type of simple task management is what you need, this is a compelling option, especially since they seem to be actively improving it.