Gnome To Do - GNOME To Do is a simplistic personal task manager designed to perfectly fit the GNOME desktop.

Todoist - Todoist is a to-do list that helps you get organized, at work and in life.

QOwnNotes - QOwnNotes is a free plain-text notepad with markdown support for Linux, Mac OS X and Windows, that works together with ownCloud Notes.

Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.

EssentialPIM - EssentialPIM is a free Personal Information Manager that keeps up with the times and lets you manage appointments, tasks, notes, contacts, password entries and email messages across multiple devices and cloud applications.

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.