MailPress Allows you to send beautiful and styled html and plain text mails based on dedicated themes and templates. Using this WordPress plugin, you can send any e-mail notification issued for comments subscribers, your periodic newsletters (per post/daily/weekly/monthly) and some specific admin events : registration of a new user, comment to moderate, new comment on your posts.

Several addons are available as zip files in the mailpress folder. They are in fact WordPress plugins interacting with MailPress. The basic idea is to keep the core of MailPress as stable as possible and to use the WordPress API to propose some add ons, making MailPress completely scalable to your needs.

Pricing: Free
Requirements: WordPress 2.6+
Source: http://wordpress.org/extend/plugins/mailpress/