org-mode of course. it's a secret weapon of Emacs. Very few people move to Emacs for org-mode, not too many emacs users use org, and the number of people who actually possess deep knowledge of org mode features is very small. org mode is truly massive. The way it changed my workflow and many others is indescribable. Anyone who uses Emacs should learn org.
Ace Jump -- You can move your cursor to ANY position ( across window and frame ) in emacs by using only 3 times key press.