Send user's password in mail
You cannot get the user's password from what Drupal saves in the database. Drupal doesn't encrypt the password and saves it in the database, but it saves in database the hash of the password, which is the value returned from a one-way function; this means that is not possible to calculate the value that originated the hash.
That is the reason why hashes are saved instead of the passwords: If somebody access the database used by a Drupal site, it is not possible to retrieve the passwords, and access that site as one of the users registered in that site. (It could still possible to find a word with the exact same hash; so far, collision attacks are shown possible in short time only with MD5.)

Drupal is able to send an email to reset a user's password, which is not what you are asking for, but it could be helpful if you want the users to reset their password after X months. In this case, you could be interested to the code used by _user_mail_notify().

  // By default, we always notify except for canceled and blocked.
  $default_notify = ($op != 'status_canceled' &amp;&amp; $op != 'status_blocked');
  $notify = variable_get('user_mail_' . $op . '_notify', $default_notify);
  if ($notify) {
    $params['account'] = $account;
    $language = $language ? $language : user_preferred_language($account);
    $mail = drupal_mail('user', $op, $account-&gt;mail, $language, $params);
    if ($op == 'register_pending_approval') {
      // If a user registered requiring admin approval, notify the admin, too.
      // We use the site default language for this.
      drupal_mail('user', 'register_pending_approval_admin', variable_get('site_mail', ini_get('sendmail_from')), language_default(), $params);
    }
  }