For Drupal6 , it is recommended to use menu_get_object()
<?php
$node = menu_get_object();
$story = $node->type == 'story';
?>
However in Drupal 7, node_load() also caches the current node object and in many cases, node_load() returns an object faster than menu_get_object()
--
http://www.thecarneyeffect.co.uk/menugetobject-vs-nodeload-when-getting-node-object-drupal-7
https://api.drupal.org/api/drupal/includes%21menu.inc/function/menu_get_object/7