Find ID of Top-Most Parent Page

This will find what the ID is of the top-most parent Page, in a nested child page. For example, this page you are literally looking at is nested under

<?php

if ($post->post_parent)	{
	$ancestors=get_post_ancestors($post->ID);
	$root=count($ancestors)-1;
	$parent = $ancestors[$root];
} else {
	$parent = $post->ID;
}

?>

$parent will be the correct ID. For example, for use with wp_list_pages.

Source: CSSGlobe