Rails Lightweight Stack. Most of this is detailed on Crafting Rails Applications - http://pragprog.com/book/jvrails/crafting-rails-applications

config.ru

# Run this file with `RAILS_ENV=production rackup -p 3000 -s thin`
# Be sure to have rails and thin installed.
require "rubygems"

# We are not loading Active Record, nor the Assets Pipeline, etc.
# This could also be in your Gemfile.
gem "actionpack", "~> 3.2"
gem "railties",   "~> 3.2"

# The following lines should come as no surprise. Except by
# ActionController::Metal, it follows the same structure of
# config/application.rb, config/environment.rb and config.ru
# existing in any Rails 3 app. Here they are simply in one
# file and without the comments.
require "rails"
require "rails/all"

class MyApp < Rails::Application
  routes.append do
    match "/hello/world" => "hello#world"
  end

  # Enable cache classes. Production style.
  config.cache_classes = true

  # Here you could remove some middlewares, for example
  # Rack::Lock, AD::Flash and AD::BestStandardsSupport below.
  # The remaining stack is printed on rackup (for fun!).
  # Rails 4 will have config.middleware.api_only! to get
  # rid of browser related middleware.
  config.middleware.delete "Rack::Lock"
  config.middleware.delete "ActionDispatch::Flash"
  config.middleware.delete "ActionDispatch::BestStandardsSupport"

  # We need a secret token for session, cookies, etc.
  config.secret_token = "49837489qkuweoiuoqwehisuakshdjksadhaisdy78o34y138974xyqp9rmye8yrpiokeuioqwzyoiuxftoyqiuxrhm3iou1hrzmjk"
end

# This is a barebone controller. One good reference can be found here:
# http://piotrsarnacki.com/2010/12/12/lightweight-controllers-with-rails3/
class HelloController < ActionController::Metal
  include ActionController::Rendering

  def world
    render :text => "Hello world!"
  end
end

# Initialize the app
MyApp.initialize!

# Print the stack for fun!
puts ">> Starting Rails lightweight stack"
Rails.configuration.middleware.each do |middleware|
  puts "use #{middleware.inspect}"
end
puts "run #{Rails.application.class.name}.routes"

# Run it!
run MyApp