Remember The Milk (http://rememberthemilk.com) is a very powerful and useful task list manager that can be integrated in Google Calendar.

Some of the great features include:

- SMS reminders of due tasks
- Tagging of tasks for easy retrieval
- Adding tasks via email (even better if you use this with http://jott.com)
- Custom task folders
- Custom task searches (e.g. show all tasks that require less than 20 minutes to complete, show all tasks due this week, etc)