CodeMirror

CodeMirror is a versatile text editor implemented in JavaScript for the browser.
It is specialized for editing code, and comes with a number of language modes and addons that implement more advanced editing functionaly.

A rich programming API and a CSS theming system are available for customizing CodeMirror to fit your application, and extending it with new functionality.

Screenshot:

Site: http://codemirror.net

Download: codemirror.zip
GitHub: https://github.com/Awilum/monstra-cms-codemirror