Taskwarrior - Taskwarrior is an ambitious project bringing sophisticated capabilities to a simple and elegant...
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
SSuite Sticky To Do - SSuite's Sticky To Do app gives you an easy way to track your tasks and organize your day.
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.
Todour - Todour is an application for handling todo.
Getting Things GNOME - Getting Things Gnome! is an organizer for the GNOME desktop environment.