Lightweight AJAX Drupal entry point. There is also https://drupal.org/project/js module for this purpose.

ajax.php

<?php
/**
 * @file
 * Custom ajax handler for custom_fast_ajax module.
 */
define('DRUPAL_ROOT', getcwd());

require_once DRUPAL_ROOT . '/includes/bootstrap.inc';
// Include utils.
require_once DRUPAL_ROOT . '/includes/common.inc';
// Light bootstrap.
drupal_bootstrap(DRUPAL_BOOTSTRAP_SESSION);
// Our module.
require_once drupal_get_path('module', 'custom_fast_ajax') . '/custom_fast_ajax.module';

$mode = check_plain($_GET['mode']);
$results = array('mode' => $mode, 'data' => array());
if ($mode == 'some_check_here') {
  _custom_fast_ajax_fast_ajax($results);
}

drupal_json_output($results);