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I think the best way to currently do this is to take the RSS for the module you want people to be notified of (if it has an RSS feed) and add set up an account on
Feedburner.com.
One of the features they offer is the ability for you to create an e-mail sign up form for when there are updates. The person does not have to be a registered member of your site.
Once you set up the account at Feedburner, look for the "Publicize" tab. You'll find a link to create an email form for your site and a few other handy, little tools that are perfect for syndicating your site.
It works out great for all the sites I use it on and I don't need to create newsletters anymore.