Getting $head_title in my module
$head_title is one of the variables passed to the html.tpl.php template file, and set from template_preprocess_html().

  // Construct page title.
  if (drupal_get_title()) {
    $head_title = array(
      'title' =&gt; strip_tags(drupal_get_title()), 
      'name' =&gt; check_plain(variable_get('site_name', 'Drupal')),
    );
  }
  else {
    $head_title = array('name' =&gt; check_plain(variable_get('site_name', 'Drupal')));
    if (variable_get('site_slogan', '')) {
      $head_title['slogan'] = filter_xss_admin(variable_get('site_slogan', ''));
    }
  }
  $variables['head_title_array'] = $head_title;
  $variables['head_title'] = implode(' | ', $head_title);


That variable would be available as $variables['head_title'], if your module implements hook_preprocess_html() or hook_process_html(). Otherwise, you just need to execute similar code to the one used by template_preprocess_html(). To adapt that code for a module, outside hook_preprocess_html() and hook_process_html(), I would rewrite it as follows.

  // Construct page title.
  if (drupal_get_title()) {
    $head_title = array(
      'title' =&gt; strip_tags(drupal_get_title()), 
      'name' =&gt; check_plain(variable_get('site_name', 'Drupal')),
    );
  }
  else {
    $head_title = array('name' =&gt; check_plain(variable_get('site_name', 'Drupal')));
    if (variable_get('site_slogan', '')) {
      $head_title['slogan'] = filter_xss_admin(variable_get('site_slogan', ''));
    }
  }

  // Remove the following line if you just need the string.
  $head_title_array = $head_title;

  $head_title = implode(' | ', $head_title);