wp_nav_menu_items filter example

gistfile1.php

<?php
function theme_menu_append( $items ) {
	$prepend  = "";
	$append   = "";
	
	$prepend .= "<li>First item always!</li>";
	
	$append  .= "<li>This is the 2nd to last item</li>";
	$append  .= "<li>This is the very last item</li>";
	
	$items = $prepend . $items . $append;
	
	return $items;
}

add_filter( 'wp_nav_menu_items', 'theme_menu_append' );