Dynamic drush aliases. Just modify $paths to include the directory to your drupal installs, and you have instant aliases.

aliases.drushrc.php

<?php
// the current environment
$env = $_ENV['ENVTYPE'] ? $_ENV['ENVTYPE'] : '';

// paths to search for drupal sites
$paths = array(
  '/var/www/multisites/',
  '/var/www/schoolyard/',
);

$new_aliases = array();
foreach ( $paths as $path ) {
  $dh = opendir($path);
  while ( $file = readdir($dh) ) {
    $fullname = "$path$file";

    // skip files, current directory, and parent directory
    if ( strpos($file, '.') === 0 || ! is_dir($fullname) ) continue;

    // look for sites or webroot/sites directories in path
    $has_webroot = file_exists($fullname . '/webroot/sites/');
    $has_sites = file_exists($fullname . '/sites/');
    if ( $has_webroot || $has_sites ) {
      $sites = $fullname . ($has_sites ? '/sites/' : '/webroot/sites/');
      $sh = opendir($sites);

      // look for domain.tld directory in sites
      while ( $site = readdir($sh) ) {
        $sitepath = $sites . $site;
        // skip ./, ../, all/, and default/ directories
        if ( is_dir($sitepath) && ! in_array($site, array('default', '.', '..', 'all')) ) {
          // construct the site alias from the environment and domain
          $new_aliases[$file][$site] = array(
            'root' => $fullname . ($has_webroot ? '/webroot/' : '/'),
            'uri' => (strpos($fullname, 'schoolyard')  ? "http://$site" : "http://$env.$site"),
          );
        }
      }
    }
  }
}

foreach ( $new_aliases as $file => $sites ) {
  // single site drupal install
  if ( count($sites) == 1 ) {
    $aliases[$file] = reset($sites);
  }
  // multisite drupal install
  else {
    foreach ( $sites as $name => $site ) {
      list($prefix, $suffix) = explode('.', $name);
      $aliases[$file . "." . $prefix] = $site;
    }
  }
}