Task Coach is an Open Source to-do manager capable of tracking all types of tasks and to-do lists.

It is designed for composite tasks, and also offers effort tracking, categories, notes and more. With Task Coach you will be able to efficiently manage all of your tasks, projects as well as any important events you add to the app. It permits you to create any type of to-do list from the super-simple to the complex. Complex lists can contain subtasks, a listing of efforts involved, cost, and partial task completions.

Task Coach comes in a very simple-to-navigate UI, but it can be customized to suit your specific need via column headers, color-coded tasks for easy reference, multiple font & icon choices, list filters and so much more.

Task Coach Features:

Creating, editing, and deleting tasks and subtasks.
Tasks have a subject, description, priority, start date, due date, a completion date, and an optional reminder. Tasks can recur on a daily, weekly or monthly basis.
Tasks can be viewed as a list or as a tree.
Tasks can be sorted by all task attributes, e.g. subject, budget, budget left, due date, etc.
Several filters to e.g. hide completed tasks or view only tasks that are due today.
Tasks can be created by dragging an e-mail message from a mail user agent (Outlook, Thunderbird, Claws Mail, Apple Mail) onto a task viewer.
Attachments can be added to tasks, notes, and categories by dragging and dropping files, e-mail messages, or URL's onto a task, note or category.
Task status depends on its subtask and vice versa. E.g. if you mark the last uncompleted subtask as completed, the parent task is automatically marked as completed too.
Tasks and notes can be assigned to user-defined categories.
Settings are persistent and saved automatically. The last opened file is loaded automatically when starting Task Coach.
Tracking time spent on tasks. Tasks can have a budget. Time spent can be viewed by individual effort period, by day, by week, and by month.
The Task Coach file format (.tsk) is XML.
Tasks, notes, effort, and categories can be exported to HTML and CSV (comma separated format). Effort can be exported to iCalendar/ICS format as well.
Tasks, effort, notes, and categories can be printed. When printing, Task Coach prints the information that is visible in the current view, including any filters and sort order.
Task Coach can be run from a removable medium.