An extensible, customizable, free/libre text editor — and more.
Ranked in these QuestionsQuestion Ranking
Pro Completely free
Licensed under GNU GPL.
Pro Keyboard-focused, mouse-free editing
Emacs can be controlled entirely with the keyboard.
Pro Infinitely customisable
Customizations can be made to a wide range of Emacs' functions through a Lisp dialect. 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 Works in terminal
You can use Emacs' command line interface or graphical user interface.
Pro Has turn-key packages for IDE work
Packs like spacemacs make it easy to get started and bring the learning curve down from an infinitely regressing spiral to something more manageable.
Pro Self documenting
Pro Has Vim emulation
Evil-mode makes Emacs actually usable as an editor.
Con Not set up as an IDE by default
Requires customization to get IDE-like features. Luckily a few features such as compilation, debugging, and syntax highlighting are included.
Con Learning curve is steep
While it's better than it used to be, with most functions being possible through the menu, Emacs is still quite a bit different from your standard editor. You'll need to learn new keyboard shortcuts.
Con Chorded keyboard combinations can be baffling
For example, for navigation it uses the b, n, p, l keys. Which for some people may seem strange.