Emacs

GNU Emacs is an extensible, customizable, free/libre text
editor — and more.

At its core GNU Emacs is an interpreter for Emacs Lisp, a
dialect of the Lisp programming language with extensions to
support text editing.

The features of GNU Emacs include:
* Content-aware editing modes, including syntax coloring, for many file types;
* Complete built-in documentation, including a tutorial for new users;
* Full Unicode support for nearly all human scripts;
* Highly customizable, using Emacs Lisp code or a graphical interface;
* A wide range of functionality beyond text editing, including a project planner, mail and news reader, debugger interface, calendar, IRC client, and more;
* A packaging system for downloading and installing extensions.

Android notes:
* System keyboard can be invoked by tapping on a non read-only buffer;
* Permissions are not granted by default, they must be explicitly enabled in system settings menu;
* Android version of emacs is built without GNUTLS support;