MailPress is a WordPress plugin by andre renaut with features enabling you to:
Style your html and plain text mails with dedicated themes and templates.
Double opt-in subscription.
Subscriptions to Comments, Newsletters/Post notifications and even to Mailing lists.
Newsletters/Post notifications on a per post, daily, weekly, monthly basis.
Optional : full control on all mails sent by WordPress.
Download and info: http://wordpress.org/extend/plugins/mailpress/