gistfile1.rb

::AppConfig = ApplicationConfiguration.new("main.yml", "main_local.yml")
AppConfig.use_environment!(Rails.env)

## main.yml
  defaults: &defaults
    one: won
    two: too
    web_service:
      url: http://foo
      port: 80

  development:
    <<: *defaults

## main_local.yml
  web_service:
    port: 8080



#### My app_config lib

class AppConfiguration
  class << self
    CONFIG_BASE = File.join(RAILS_ROOT, 'config', 'app_config')

    def app_config_configuration(environment)
      configs = load_config_file(app_config_configuration_file)
      hashes  = [configs['default'], configs[environment]]

      begin
        local_config = load_config_file(app_config_override_file)
        hashes << local_config
      rescue
        nil ### local config optional.
      end

      hashes.compact!
      hashes.inject({}){|aggregate,current| recursive_merge(aggregate, current)}
    end

  protected
    def load_config_file(file)
      YAML::load(ERB.new(IO.read(file)).result(binding))
    rescue => e
      raise "ERROR: loading '#{file}': #{e}"
    end

    def app_config_configuration_file
      "#{CONFIG_BASE}.yml"
    end

    def app_config_override_file
      "#{CONFIG_BASE}_local.yml"
    end

    ##
    # Taken from http://github.com/cjbottaro/app_config/tree/master
    def recursive_merge(h1, h2)
      h1.merge(h2){|k, v1, v2| v2.kind_of?(Hash) ? recursive_merge(v1, v2) : v2}
    end
  end
end