In this new tutorial to the TrueLogic.org site, they show you how to use the Mailchimp (using a library) from your application.

For those unfamiliar with Mailchimp it is an online service for handling subscribers and newsletters. It is mainly used to handle subscription lists and sending newsletters via its Mailchimp API which has wrappers and integration features for various languages/platforms. [...] In this article we will take a quick look at how to integrate this with your PHP application. If you want a detailed reference on integration then its better to look at the official API documentation. Here, we just want to give a simple and practical explanation on how to go about it.

Once you've gotten an API key, you can follow along with the tutorial. They show you how to subscribe to lists, create campaigns, unsubscribe from a list and delete a campaign.

On the Web Development Blog there's a recent post looking at the integration of the MailChimp mailing list service with your current application for list management.

MailChimp is a great mailing list provider with a great control panel and many useful features like: campaign management, statistics, responders and complete set of List management tools. They offer also an small code snippet to place a subscription form on your website. This form powered by jQuery is good enough for most common cases, but what if you like to combine your newsletter subscription with a standard function like contact or registration form?

They show you how to create the front end - a HTML form pointing to your own PHP script - and the PHP code to make a cURL request over to the MailChimp servers to subscribe the given email address to your list.