There is a great tutorial on advanced features for Gmail on the Become a Gmail ninja site. (Editor note: Wayback Machine latest archive that shows the intended content).
What are some other tips or little known features of Gmail that you use which help you stay organized, and which do not appear on the Become a Gmail ninja site?
Please only post answers that do not appear in the become a Gmail ninja list, I do not want to encourage a duplicate list of content.