rails_app.rb

# First, enable "Listen for incoming notifications" 
# in Growl preferences and set a password

# You might want to use this in an initializer

growl = Growl::GNTP.new 'ip_of_your_mac', 'App name'
growl.password = 'your_password'

# growl.add_notification 'name'
# growl.notify 'name', 'title', 'message'

ActiveSupport::Notifications.subscribe 'hook_name' do |*args|

  event = ActiveSupport::Notification::Event.new args

  if event.payload[:some_key] = 'some_value'
     growl.notify 'notification', 'title', 'message'
  end

end

stand_alone_app.rb

require 'ruby-growl'

growl = Growl::GNTP.new 'ip_of_your_mac', 'App name'
growl.password = 'your_password'
growl.notify 'notification', 'title', 'message'