Numinix developed this plugin for MailChimp.com using the v3.0 API and offers installation services.
Get all your Zen Cart customers into MailChimp automatically. When an order is placed, the module checks to see if they are already using MailChimp. If so, their purchase history is updated and their current subscription status is maintained. If not, they are subscribed and the order information is also added.
Sync all of your existing subscribers from Zen Cart to Mailchimp.com via the Mailchimp Sync Tool in the Zen Cart admin.