I'm currently working on a newsletter module for Siena, and hopefully some people want to try it out.
Please note that I am still very much learning PHP and Cotonti. This is my first module so please bare with me and feel free to point out problems or improvements. :)

Features:

• It will only send the newsletter to subscribed users. Users can simply subscribe to the newsletter by ticking a checkbox in their profile.
• Admins can easily create a newsletter with for example CKEditor.
• When creating a newsletter an admin can choose to first send a test version to the administrator's email address, before mass sending.
• Every newsletter will have the user's name in the salutation.
• Every newsletter delivered to the user will have an unsubscribe link which they can click to unsubscribe from the newsletter.
• The newsletters are kept in the database, so you can easily view a history of the newsletters that were sent..

Installation:

1. Drop the newsletter folder in your Cotonti's module folder.
2. Install the module in your administration panel.
3. The installer will add an extrafield to the database, you need to add this extrafield to the users.profile.tpl file.
   The field is: {USERS_PROFILE_NEWSLETTER}

Download:
http://www.twiebie.com/cotonti/modules/newsletter.rar

Note: Do NOT use this module on a live website, it has not fully been tested yet and proper auth stuff is not yet in place.