I use http://www.phplist.com/ - if you have MySQL databases on your server you can use this. There is a bit of a learning curve to set it up, but once you've figured it out (and there are good forums) you can add a subscribe box to your page that automatically deals with the subscription, sends them opt-in emails (so you are canspam compliant) and allows you to send out messages to the group. Hope that helps!