You need to get an autoresponder if your interested in successful email marketing. I would suggest emailaces as they have an excellent price for a professional service. Of course aweber or constant contact or get response are all fine services as well...just a little higher priced. If you just want to send an occasional email to your list then you might like phplist. This script will allow you to put a signup form on your site and send broadcast emails to your list. This is NOT an autoresponder, it is a very robust and feature filled mailing list management tool that is absolutely free to download, install, and use.
If you are referring to creating the functionality of a newsletter and not the layout of the actual message then I recommend using Google Groups: - Users can subscribe through a form that you can post on your site - Google handles all the email traffic - You send a message to all users by submitting a form at google groups page or by sending an email to your_group_name[at]googlegroups.com - Adrian.