If you have a MailChimp account, you can easily integrate this service using the/scripts/mailchimp/subscription.php file. Don’t forget to add your account info on lines 7 and 11 and then move the 2 files from /scripts/mailchimp/ to /scripts/. Obviously, this will replace the regular subscription.php file.

These 2 files you need to copy are MCAPI.class.php and subscription.php

Remember that to make mailchimp work, you need to use your Mailchimp API + Mailchimp list ID...(not your username)

In the file: /scripts/mailchimp/subscription.php
you will find:

// Get your API key from http://admin.mailchimp.com/account/api/
define('MC_API_KEY', 'enter you mailchimp api key here');
// Get your list unique id from http://admin.mailchimp.com/lists/
// under settings at the bottom of the page, look for unique id 
define('MC_LIST_ID', 'enter you mailchimp unique list id');

After editing, save it and then you need to move this file AND MCAPI.class.php into your /scripts/ folder (so it will replace/overwrite /scripts/subscription.php)