/**
 * Implements hook_menu().
 */
function mymodule_menu() {
  $items['user/%user/mytab'] = array(
    'title' => 'My Tab',
    'page callback' => 'mymodule_mytab',
    'page arguments' => array(1),
    'access arguments' => array('access content'),
    'type' => MENU_LOCAL_TASK,
    'weight' => 10,
  );
  return $items;
}
 
/**
 * Page callback
 */
function mymodule_mytab($user) {
  return '<pre>' . htmlspecialchars(print_r($user, true)) . '</pre>';
}

Drupal 6, Drupal 7