From http://stackoverflow.com/questions/3705898/best-way-to-add-current-class-to-nav-in-rails-3

nav_link_helper.rb

def nav_link(link_text, link_path)
  class_name = current_page?(link_path) ? 'current' : nil

  content_tag(:li, :class => class_name) do
    link_to link_text, link_path
  end
end

# In the view (example)
<%= nav_link 'Home', root_path %>