Task Coach - Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists.

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.

GTDNext - About Us.

CherryTree - A hierarchical note taking application, featuring rich text and syntax highlighting, storing data in a single xml or sqlite file.

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