Task Coach

Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists. It is designed for composite tasks, and also offers effort tracking, categories, notes and more.

http://taskcoach.org/

ToDoList

An effective and flexible way to keep on top of your tasks

ToDoList is a rare form of task management tool, one that allows you to repeatedly sub-divide your tasks into more manageable pieces whilst still presenting a clean and intuitive user experience.
ToDoList has been in continuous development for the last 6 years and is an ongoing project.
Your tasklists are stored in XML which provides many opportunities for advanced formatting and printing using stylesheets.
ToDoList's flexible design makes it ideal for both IT related projects as well as more general GTD uses. 

http://www.abstractspoon.com