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

Gemfile:

gem 'ace-rails-ap'

application.js:

//= require ace/ace //= require ace/worker-html

To include a theme or mode, put:

//= require ace/theme-sometheme //= require ace/mode-somemode

...in your application.js manifest as well.

Then just use Ace like normal.