app_config.rb

class AppConfig
  class << self
    attr_accessor :site_name
    attr_accessor :gateway
    ...
  end
end

development.rb

config.after_initialize do
  AppConfig.gateway = ActiveMerchant::Billing::AxiarGateway.new(:login => 'test', :password => 'test', :mode => self.gateway_mode)

end

whatever.rb

...

require 'app_config'

AppConfig.site_name = 'My Site'
..