external-admin-link.php

<?php

//Plugin Name: External Admin Link

new External_Admin_Link();

class External_Admin_Link {

	function __construct() {
		add_action( 'admin_menu', array( &$this, 'menu' ) );
	}

	function menu() {
		global $menu;

		// array( $name, $capability, $link, $notsure, $classes, $slug_ithink, $notsure )
		// $slug_ithink would only be relevant for "real" admin pages

		$new_link = array( 'Test', 'read', 'http://google.com', '', 'menu-top menu-icon-settings', '', 'none' );
		array_push( $menu, $new_link );
	}

}