forgot_username.module

<?php
function forgot_username_menu() {
  $items['user/username'] = array(
    'title'            => 'Forgot your Username?',
    'page callback'    => 'drupal_get_form',
    'page arguments'   => array('forgot_username_form'),
    'access arguments' => array('access content'),
    'description'      => 'Allows a user to submit their email to receive their username',
    'type' => MENU_CALLBACK,
  );
  return $items;
}

/**
 * generate the form where the user requests their username
 *
 * @return
 *   The settings form used by the user to request their username
 */
function forgot_username_form() {
  $form['email_address'] = array(
    '#type' => 'textfield',
    '#title' => t('Your email'),
    '#description' => t("We will send your username to your email"),
  );
  $form['submit'] = array(
    '#type' => 'submit',
    '#value' => t('Request Username'),
  );
  $form['#submit'][] = 'forgot_username_form_submit';
  return $form;
}

/**
 * Form validation handler.
 */
function forgot_username_form_validate($form, &$form_state) {
  if(!valid_email_address($form_state['values']['email_address']))
    form_set_error("email_address", "Inavlid email address");
}


/**
 * Form submission handler.
 */
function forgot_username_form_submit($form, &$form_state) {
  $query="select name from {users} where mail = '%s'";
  $result=db_query($query, $form_state['values']['email_address']);
  $user=db_fetch_array($result);
  if(!$user)
    form_set_error('email_address', 'There is no account with that address');
    else  {
     
      $message = array(
        'to' => $form_state['values']['email_address'],
        'subject' => t('Your requested username'),
        'body' => t('Your username is: '.$user['name']),
        'headers' => array('From' => 'admin@arcsfoundation.com'),
      );

drupal_mail_send($message);
drupal_set_message('We have sent your username to your email');
drupal_goto(drupal_get_normal_path(variable_get('site_frontpage', 'node')));
    }
}