Ajaxified MailChimp form
This plugin should work right out of the box with the default html MailChimp signup form with 1 modification to the form action url
//XXXXX.usX.list-manageX.com/subscribe/post-json?u=XXXXXXXXXX&id=XXXXXXXX&c=?
In order to work you will need to add -json after post and &c=? at the end of the url