SharePartXXL (www.sharepartxxl.com) just offer a new product called SharePartXXL Newsletter Webpart for Microsoft SharePoint which allows you to completely manage newsletters directly in SharePoint (WSS/MOSS). All data for newsletter types, newsletters, subscribers, subscriptions etc. is managed in default Sharepoint lists, so it is very easy to integrate and to customize. For example all columns that you create in the lists can be used as placeholders inside the newsletter templates. There are web parts for subscription workflow (double-opt-in) and sending available.
If you are interested in finding out more about the SharePartXXL Newsletter Webpart for Microsoft SharePoint then please check
http://www.sharepartxxl.com/products/newsletter/
and download the free evaluation version or email to sales@sharepartxxl.com .