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WP-AJAX Newsletter
Housed at Google code, apparently this plugin allows for a newsletter to be emailed to all registrants of blog content. Lots of ability to customize the outgoing newsletter, as well as an AJAX powered registration (including email conformation for registration). I couldn’t decipher if you could include content outside of blog posts or not.
Instinct Entertainment
According to the authors “WP Campaign Monitor is an email newsletter / SMS tool built for WP users who want to send campaigns, track the results and manage their subscribers. Version Two will integrate with e-commerce lite.” I haven’t tested this, but there very well is a demand for such plugins, especially for those developing WP more as a CMS. While there, checkout the e-commerce plugin. A lot of development has been done since its first introduction.