This addon provides integration with Mailchimp allowing your users to sign up for mailing lists. It uses the new version 3 Mailchimp API to do the hard work. Submission is handled by Ajax, so your visitors never leave the page they're on. The addon installs a block that you add to your page which adds a form, allowing your visitors to sign up to your mailing lists.
You will need to sign up for a Mailchimp account, and you will need to generate an API key which you can do at https://admin.mailchimp.com/account/api/
The addon will retrieve the mailing lists associated with your account and will offer a choice of lists, or you can allow the user to choose which lists they want to subscribe to. You can control whether they pick just one list via a dropdown, or from all lists using checkboxes.
You can choose whether to allow the user to choose between html and plain text format, or choose a value for them. You can allow the user to enter their first and last names too.
Note that this addon requries that your webserver must be able to access the internet.