phplist (open source)
phplist is a PHP based open source application that can be installed on your domain to manage newsletters and other unidirectional email communications.
This application requires basic knowledge of file editing and HTML/CSS design to properly create HTML messages to send out, but the instructions are very helpful and straightforward.
FastQ customers familiar with FTP and basic web application installation procedures to install, customize and manage your mailing lists.
Download phplist: http://www.phplist.com
phplist Documentation: http://docs.phplist.com/Documentation