The Ajax.org Cloud9 Editor (Ace) for the Rails 3.1 asset pipeline.
Gemfile:
gem 'ace-rails-ap'application.js:
//= require aceTo include a theme or mode, just put:
//= require theme-[sometheme].js
//= require mode-[somemode].jsAfter //= require ace in your application.js manifest.
Then just use Ace like normal.