Mail Chimp is an email marketing system that is free to begin with, but you will start paying once you reach 500 subscribers.
Aweber, on the other hand, costs even if you have no subscribers to start with, but has more powerful tracking and email management features compared to MailChimp.
We recommend that you use either MailChimp or Aweber right from the moment you launch your website. It’s the most effective way to keep your customers in the loop, and the more subscribers you gain, the wider your net for making sales or for marketing.