You could either create constants in your config/environment.rb file (or config/environments/{development, test, production}.rb) which you can refer to elsewhere in your code, or you could load, say, a YAML file in config/environment.rb and assign the YAML to a constant.
Here's an example of the latter:
http://www.realityforge.org/articles/2006/02/28/flexible-application-configuration-in-rails
Regards, Andy Stewart