#Behold, the nav_link:
The nav_link helper works just like the standard Rails link_to helper, but adds a 'selected' class to your link (or its wrapper) if certain criteria are met. By default, if the link's destination url is the same url as the url of the current page, a default class of 'selected' is added to the link.
For full usage details, see: http://viget.com/extend/rails-selected-nav-link-helper
Drop nav_link_helper.rb into app/helpers in your Rails 3.x app and enjoy.