./config/my_configuration.yml

MyConfig:
 username: my_username
 password: my_password
 host: my_host

./config/environment.rb

require 'ostruct'
require 'yaml'

OwnConfigFile = "#{RAILS_ROOT}/config/my_configuration.yml"
if File.exist?(MyConfigFile)
  ::ApplicationConfig = OpenStruct.new(YAML.load_file(MyConfigFile))
end

in application:

config = ApplicationConfig.MyConfig;
puts config['username']