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)
To get the plugin either go to wordpress.org or see the plugin’s page on this site here.