A comprehensive description of how I currently use org-mode.
One of the godfathers of text editors; free long-standing software project; tons of extensions and funcionalities; the biggest drawback (my taste): it needs E-Lisp for being programmed.
GNU TeXmacs is a free wysiwyw (what you see is what you want) editing platform with special features for scientists. The software aims to provide a unified and user friendly framework for editing structured documents with different types of content (text, graphics, mathematics, interactive content, etc.). The rendering engine uses high-quality typesetting algorithms so as to produce professionally looking documents, which can either be printed out or presented from a laptop.
Org mode is for keeping notes, maintaining TODO lists, planning projects, and authoring documents with a fast and effective plain-text system.
Super-powerful Emacs plugin to manage outlines with associated timestamps, priorities, labels, etc.; available views grouped by time (agenda), tags, etc.; plain text storage format.