Are you talking about a plugin to let people sign up for your newsletter? If that's what you're after, let me tell you - it doesn't exist. There are a few plugins out there, but none of them look good at all. What I'd suggest doing is creating a signup form in your Mailchimp account, then grabbing the form code and putting it in a sidebar widget or even embedding it in your theme. You can then create an image background for it and use CSS to position the form elements correctly. Check out one of my blog posts to see what I mean - I've got a signup form in the sidebar and directly below the post.
Let me know if you need any help styling your forms.