Squirrel HackerI have come forward in time from the year 1986 to share this with you

<?php
/**
 * Capture the form stubmit and check the form_id if it matches a login form, then
 * it will check it as an email using an extra validation step
 * @param array $form The current form  (I guess...)
 * @param array $form_state The state of the current form  (I guess...)
 * @param string $form_id The unique identifier for this form
 * @see hook_form_alter
 */
function email_login_form_alter(&$form, $form_state, $form_id) {
  switch ($form_id) {
    //there are two normal login forms
    case 'user_login':
    case 'user_login_block':
      // Ensure a valid validate array.
      $form['#validate'] = is_array($form['#validate']) ? $form['#validate'] : array();
      // your validation function must run first.
      array_unshift($form['#validate'],'email_login_user_login_validate');
      break;
    //ignore other stuff
    default:
      return;
      break;
  }
}
 
/**
 * Capture the username entered, if there is an @ symbol, it will check it against email addresses instead of the username
 * if multiple_email is installed, check against that table instead
 * @param array $form The current form  (I guess...)
 * @param array $form_state The state of the form (I guess...)
 */
function email_login_user_login_validate($form, &$form_state) {
  if (isset($form_state['values']['name']) && strpos($form_state['values']['name'], '@') !== false) {
    if(module_exists('multiple_email')){
        $name = db_result(db_query("SELECT users.name FROM {users} JOIN multiple_email ON multiple_email.uid = users.uid WHERE (LOWER(multiple_email.email) = LOWER('%s') AND multiple_email.confirmed = TRUE) OR LOWER(users.mail) = LOWER('%s')", $form_state['values']['name'], $form_state['values']['name']));
    } else {
        $name = db_result(db_query("SELECT name FROM {users} WHERE LOWER(mail) = LOWER('%s')", $form_state['values']['name']));
    }
    if ($name) {
      form_set_value($form['name'], $name, $form_state);
    }
  }
}