Email marketing is one of the most common ways to reach out and "ping" people to remind them that you exist. It is used for existing customers as much as for prospective customers and it's a relatively cheap way to stay "front of mind".
Mailchimp is similar to other email marketing systems like Aweber, CampaignMonitor, Get Response and many more and it enables you to use powerful features for free until you reach a good number of "subscribers".