Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Gnome To Do - GNOME To Do is a simplistic personal task manager designed to perfectly fit the GNOME desktop.
Remember The Milk - Remember The Milk is a task and time management application for mobile devices.
Org mode - Org: an Emacs Mode for Notes, Planning, and Authoring
mCheck - A checklist application written in java with reminders and system tray support to improve...
Taskade - Make lists, organize your thoughts, and be inspired to get things done. Taskade is a collaborative space for your tasks.