Auto Responders automatically send people a times series of newsletters starting the day they sign up. Once you set up a series of auto responders, it goes into autopilot.
Automated Sales Helper
Personally, I've started using SendStudio. It's not the cheapest, but it is easy to install and easy to use. If there's a feature it doesn't have, I can't think of it.
See review in www.autoresponder-reviews.com