application_helper.rb

# link_to 'Foo', bar_path, :class => active_link?('some_controller', 'some_action')
# link_to 'Foo', bar_path, :class => active_link?('some_controller') # Active anywhere within the controller
def active_link?(controller, action = nil)
  action_match = case
  when action.nil? then true
  when action.is_a?(Array) then action.include? params[:action]
  else action == params[:action]
  end

  params[:controller] == controller && action_match ? 'active ' : ''
end