BroadFast Lite is simple FREE software under GPL license.

It allows you to send newsletters to the users in your mailing list.

If you are looking for more advanced features like custom fields, SMTP or Sendmail sending, handling large mailing lists, customization of the emails, read/click trackind, attachments and more, check out BroadFast.

It is very affordable, and comes with free support and money-back guarantee.