Quoted : phplist is an open-source newsletter manager. phplist is free to download, install and use, and is easy to integrate with any website.

Quoted: PHPList is a excellent open source newsletter manager. However, one missing piece from the application is the ability to cleanly and easily allow users to subscribe to your newsletter from your wordpress blog. This lightweight plugin for Wordpress 2.0 or 2.1 gives you the ability to easily allow users to subscribe to your newsletter or RSS blog feed from any page on your blog.

This is a WordPress plugin that can help you manage your PHPList mailing lists from the WordPress admin panel. Although it’s not a stand alone plugin, it does allow for easy management, subscription, unsubscription, and mailing of your subscribers from the WordPress admin panel. It does require that you have the PHPList software installed and configured in order for it to work properly.

Quoted: The newsletter manager plugin for WordPress lets you easily integrate a third party newsletter manager to your site.

Note : Not a free plugin.

Quoted: By default, WordPress lacks the ability to send emails to registered users. This plugin was created to fill the gap by allowing you to send emails to your blog users.

Create a custom form for use with WordPress that sends an email to a specified email.

Quoted: This site provides access to the latest version of the Subscribe2 plugin for WordPress. Subscribe2 notifies subscribed users each time a new post is made .

Quoted: Allow users/members to subscribe to your Wordpress mailinglist and send out mass newsletters to all subscribed members in your database.