Menu styling for last menu item

Here’s a function I use to add a first / last class to wp_nav_menu() items:

function add_first_and_last($items) {
    $items[1]->classes[] = 'first';
    $items[count($items)]->classes[] = 'last';
    return $items;
}
add_filter('wp_nav_menu_objects', 'add_first_and_last');

Developer Resources – wp_nav_menu_objects Hook