I know this is less programming related and more time management related, but I value the feedback of the users on this site. I\'m finding myself particularly busy this semester
Use the Tasks in GMail. They are useful, prett ylightweight, you can have a hierarchy of tasks. Good if you are already using GMail.
There is also a Remember The Milk plugin for GMail. Here you can't have sub-tasks, but it's pretty good too, all in all.