Easy WordPress Mailchimp Integration is a plugin that add Mailchimp signup process to WordPress registration form. You can customize the message that is displayed on the form registration.
Its List Manager helps you build and manage huge lists of emails in a simple and hassle-free manner. You can also design and track email campaigns using MailChimp’s tools.
Plugin homepage: http://remicorson.com/easy-wordpress-mailchimp-integration/