Toodledo is a very powerful, free task manager. You can organize your tasks using folders, subtasks, due-dates, tags, and priorities. You can set up a hotlist as well as email and SMS reminders. It even has a tool that analyzes due dates and priorities to create a schedule for you to make the best use of time and get your tasks completed. 

You can access your to do list on mobile phones, email, calendar, RSS reader, IM, and integrated into your web browser. For those who still like paper, there is even a way to print a special fold able paper booklet.

You can customize your Toodledo setup, collaborate with others, and even import tasks from other systems (like Palm, Outlook, iCal, and more. 

This is a great app to get organized and is great for teachers and students.