• Task Coach

From author's site:

Task Coach is a simple open source todo manager to manage personal tasks and todo lists. It grew out of Frank's frustration that well-known task managers, such as those provided with Outlook or Lotus Notes, do not provide facilities for composite tasks. Often, tasks and other things todo consist of several activities. Task Coach is designed to deal with composite tasks.

Notes: 

Task Coach runs neatly in the System tray in Windows and has a useful Task tracking feature that enables easy tracking of time spent on each task, referred to in the program as an "Effort".