CodeMirror comes with support for 60+ languages, and addons that implement more advanced editing functionality (autocompletion, code folding, configurable key bindings, search & replace, linter integration etc.). A rich programming API and a CSS theming system are available for customizing CodeMirror, and extending it with new functionality.
It has been developed since early 2007, has a Wikipedia page, and is being used in a wide number of popular open source projects (Joomla, Firebug etc.).