I have just finished writing a simple yet useful addon plugin for Contact Form 7.
This plugin will add the people who send you a message using the Contact Form 7 to your MailChimp list.
The plugin’s administration page allows you to enter your MailChimp API key and also your list name.
You can also add a checkbox in order to give your visitors a choice whether they want to subscribe to your list or not. (see the plugin home page shown below)