The Ajax.org Cloud9 Editor (Ace) for the Rails 3.1+ asset pipeline.

Gemfile:

gem 'ace-rails-ap'

application.js:

//= require ace/ace

To include a theme or mode, just put:

//= require ace/theme-[sometheme].js
//= require ace/mode-[somemode].js

After //= require ace/ace in your application.js manifest.

Then just use Ace like normal.