Constant Contact is an email autoresponder service that's been around for several years now. It allows you to send your subscribers information, updates, other messages on a regular basis.
First you need to use a form so prospects and customers can sign up via email. Then you'll need to publish it on your website and direct your visitors to put in their email address. You can setup a follow up campaign that automatically sends messages to your prospects over time, and you can also broadcast messages in real-time.
When you review Constant Contact, you'll be able to see that their best features include: tracking, reporting, html templates, affordability, and more.
Tracking and reporting is important so you can see who is clicking on your emails, and who's clicking the links inside your emails. You'll also be able to track what follow up and broadcast subject lines are doing better than others. This will help you optimize your emails to get your more opens and clicks.
Constant Contact has a good number of html templates, and the designs are relatively worthwhile.
Constant Contact's pricing is competitive. It starts at $15/mo for up to 500 subscribers, and is usually one of the most affordable options even when you grow your subscriber base into the thousands.