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.
Remember The Milk - Remember The Milk is a task and time management application for mobile devices.
mCheck - A checklist application written in java with reminders and system tray support to improve...
Org mode - Org: an Emacs Mode for Notes, Planning, and Authoring
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.