If you're looking for a way to improve the quality of your newsletter, take a look at MailChimp (www.mailchimp.com). They offer html templates, double opt-in, archive management, multiple lists and loads more features for a very affordable price. They only charge per email (not a monthly fee like Constant Contact), so it's quite affordable for the small merchant who only sends newsletters once in a while.
Code to integrate the "subscribe" form for MailChimp as a sidebox is provided here:
Even if you're not yet ready to start a newsletter, it's worth visiting their site to check out some of the resources and guides they offer to anyone considering email marketing.
That Software Guy