Recs.
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A robust text editor capable of achieving whatever it is the writer wishes. It supports a plethora of programming languages and other faculties of text editing. Yet its advantages do not stop there. If one wishes to delve into the wizardry of Emacs, they can use it for email, web browsing, organizing ones life and so much more. With all these factors pertaining to the sovereignty of Emacs, only one question remains. What is stopping you from beholding its power?
SpecsUpdate
Pros
Pro Self documenting
Emacs has extensive help support built-in as well as a tutorial accessed with C-h t.
Pro Total customizability
Customizations can be made to a wide range of Emacs' functions through a Lisp dialect (Emacs Lisp). A robust list of existing Lisp extensions include the practical (git integration, syntax highlighting, etc) to the utilitarian (calculators, calendars) to the sublime (chess, Eliza).
Pro Org-Mode
Emacs' org-mode is great for creating any structured document type. It has LaTeX support, so you can publish books and other document-types directly from org-mode. Org-mode is included by default in Emacs and has a long list of features right out of the box such as embedded tables, code results executed in-line, url hyperlinks, images, etc... It also makes a great To-Do list!