My email inevitably reminds me of, or alerts me to things I have to do. I love Gmail's task lists which allow you to create quick and easy to-do lists right from your inbox.
Update: In the new Gmail interface, find your Tasks lists in the dropdown menu here:
(I'd like to apologize up front for the poor quality of the audio in this video — it's a bit jumpy — but I think it's still clear enough to understand.)
Can't see the video? Watch it here!