Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
WeekToDo - Free and Open Source Minimalist Weekly Planner and To-Do list App Focused on Privacy
Gnome To Do - GNOME To Do is a simplistic personal task manager designed to perfectly fit the GNOME desktop.
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
DejaOffice - Super-charged calendar, contacts, tasks and notes apps for Android, iPhone and iPad.
Tweek.so - A concise and simple weekly to do list calendar app to organize your tasks online. A lightweight weekly planner to maintain your focus and boost productivity week over week.