According the Mandrill's docs (http://help.mandrill.com/entries/21738166-API-Wrappers), mandrill-api-node is the official node client. It's not available on npm, only BitBucket:
npm install git+https://bitbucket.org/mailchimp/mandrill-api-node.git --save
However, there is also a third party wrapper they list on the above-mentioned page and also provide an example for (https://mandrillapp.com/api/docs/index.nodejs.html). This can be installed as follows:
npm install mandrill-api --save