/**
 * Реализация hook_menu()
 */
function mymodule_menu() {
  $items['node/%node/mytab'] = array(
    'title' => 'My Tab',
    'page callback' => 'mymodule_mytab_callback',
    'page arguments' => array(1),
    'access callback' => 'mymodule_mytab_check_permission',
    'access arguments' => array(1),
    'type' => MENU_LOCAL_TASK,
    'weight' => 10,
  );
 
  return $items;
}
 
/**
 * Access callback
 */
function mymodule_mytab_check_permission($node) {
  return ($node->type == 'page');
}
 
/**
 * Page callback
 */
function mymodule_mytab_callback($node) {
  return '<pre>' . check_plain(print_r($node, TRUE)) . '</pre>';
}

Drupal 6.x