Drupal 7: Drush command 'pm-lost' lists projects which are not installed or disabled.

pm_lost.drush.inc.php

<?php
/**
 * @file pm_lost.drush.inc
 *   Drush command 'pm-lost' lists projects which are not installed or disabled.
 *
 * It can be useful to detect waste modules which was downloaded once and then forgotten.
 */

/**
 * Implementation of hook_drush_command().
 */
function pm_lost_drush_command() {
  $items['pm-lost'] = array(
    'description' => 'Show a list of lost (downloaded but disabled) modules.',
    'aliases' => array('lost'),
  );

  return $items;
}

/**
 * Command callback
 */
function drush_pm_lost() {
  $modules = pm_lost_get_list();
  uasort($modules, '_drush_pm_sort_extensions');

  $rows = array();
  $header = array(dt('Name'), dt('Package'), dt('Type'), dt('Version'), dt('Status'));

  $rows[] = $header;

  foreach ($modules as $key => $module) {
    $rows[] = array(
      $module->info['name'] . ' (' . $module->name . ')',
      ($module->info['package'] ? $module->info['package'] : ''),
      ucfirst($module->type),
      @$module->info['version'],
      drush_get_extension_status($module),
    );
    $pipe[] = $module->name;
  }

  drush_print_table($rows, TRUE);
  if (isset($pipe)) {
    // Newline-delimited list for use by other scripts. Set the --pipe option.
    drush_print_pipe($pipe);
  }

  return $modules;
}

/**
 * Returns list of lost modules
 */
function pm_lost_get_list() {
  // All modules (without hidden)
  $modules = drush_get_extensions(FALSE);
  // Lost modules
  $lost_modules = array();
  foreach ($modules as $module) {

    if ($module->status == 0) {
      $still_not_lost = FALSE;

      // If it is not first level project, skip it
      if (!empty($module->info['project']) && $module->info['project'] != $module->name) {
        $still_not_lost = TRUE;
      }

      // Skip core modules like Forum
      if (!empty($module->info['package']) && $module->info['package'] == 'Core') {
        $still_not_lost = TRUE;
      }

      // If it is base theme for other theme, skip it
      if ($module->type == 'theme') {
        $all_themes = drush_get_themes();
        foreach ($all_themes as $theme) {
          if (!empty($theme->info['base theme'])) {
            if ($theme->status && $theme->info['base theme'] == $module->name) {
              $still_not_lost = TRUE;
            }
          }
        }
      }

      if (!$still_not_lost) {
        $lost_modules[$module->name] = $module;
      }

    }
  }
  return $lost_modules;
}

Thanks for this! I use it regularly to keep my Drupal installs clean.

I need it to work with Backdrop CMS too, but it had issues out-of-the-box, so I forked it and wrote my own copy. You can check it out here if you're interested: https://gist.github.com/BWPanda/c82f09d8490b5a04a8b5468e001df642