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.