Simple Drupal module to add display view modes

mymodule_view_modes.info

name = Custom Entity Display Modes
description = Adds additional display modes
core = 7.x

mymodule_view_modes.module

/**
 * Implements hook_entity_info_alter().
 *
 * Adds a new order view mode
 */
function mymodule_view_modes_entity_info_alter(&$entity_info) {
  $entity_info['node']['view modes']['custom'] = array(
    'label' => t('Custom Display'),
    'custom settings' => FALSE,
  );
}
function mymodule_view_modes_preprocess_node(&$vars) {
  if ($vars['node']->type == 'node_type' && $vars['view_mode'] == 'custom') {
    $vars['theme_hook_suggestions'][] = 'node__node_type__custom';
  }
}