The Online Outbox plugin by Chris Hutchison and Rob Riggs allows you to easily add a subscription form to your WordPress site, to grow your list of email subscribers of your Email Newsletters, Mass Email Marketing messages, Email Autoresponders, Email Surveys, Optin Emails, Email Marketing Statistics.
You can customize the form and the mail contents flexibly with simple markup. The form supports Ajax-powered submitting, CAPTCHA, Akismet spam filtering and so on.
Requires an account at http://www.OnlineOutbox.com. Accounts sending fewer than 500 emails per month are free.
http://www.onlineoutbox.com/
Download and install info: http://wordpress.org/extend/plugins/online-outbox-subscription-form/