Hey all.  We've been using Constant Contact for our newsletter at
LearnOutLoud.com.  For the most part we've been really satisfied.  We
like the fact that we don't have to handle any of the adminstration
and that Constant Contact has a lot of stuff in place to prevent our
e-mails from getting caught up in the spam filters.  However, there is
one problem and it's a major one.

When someone registers for our site
(http://www.learnoutloud.com/Register) we want them to be able to
simply check a box and subscribe to our newsletter.  However, we
haven't been able to do this with Constant Contact.  We've contacted
them and they've told us that there's no way to do this with their
service.  We're running on a PHP platform with a MySQL database.

Ideally we would like a hosted solution because of the ease of use and
anti-spam protection.  However, we would be willing to install an
application on our server if that made more sense.  We currently have
about 500 subscribers and send out HTML newsletters once a month.  We
expect both our subscriber base and the frequency of the newsletters
to increase significantly in the coming months.

Post your answer as a comment first.  If we decide to use your
solution then we will have you post it as the answer and will accept
it.

Please let me know if you have any questions.

Jon

I have found that "EMMA" works great for my Small Business.  (Well
over 500 subscribers)

http://www.myemma.com/


Topica is also a big name in the industry.  I hear people like it.

I hope this helps!