Without a doubt, Org Mode is one of the most impressive and versatile features of Emacs. Great for writing structured documents, managing lists and planning, and for exporting those documents into a variety of formats, it's my go to tool for flexible, efficient editing.
One of the requirements of my job is to send out a daily message. This message should summarize what I did the day before, and what I hope to accomplish for the current day. At first, I would spend my day jumping from task to task, and then the next morning, I would rack my brain to make sure I included all the things I worked on.
Any editor worth its salt has some provision for text snippet expansion. TextMate, Vim, and Sublime Text 2 all have this capability and Emacs is certainly no exception. If you're not familiar with the concept of snippets, the basic idea involves defining a keyword, which when followed with some trigger (keyboard shortcut or menu option), replaces that keyword with some predefined text. This functionality is a great boost to productivity as it prevents the developer from having to manually type potentially hundreds or thousands of lines of relatively boilerplate code.