The mailchimp4umbraco is a simple package that wraps the mailchimp API.
To use the package, you need a mailchimp account and a API key that can be found in your account settings.
The package comes with two default signup forms.
The simple that only needs an email addy.
The extended that needs an email addy and a name.
It also comes with a simple unsubscribe form.
All texts in the package are located in the umbraco dictionary. Custom styling of the forms is done by editing the ascx files.
An install guide can be found here: http://devblog.mediaworkers.dk/blog/mailchimp4umbraco
I hope you enjoy it :)