The free plan supports up to 12000 transactional emails per month, webhooks, up to 2000 free contacts for marketing emails and has a nice NodeJS API with a good documentation (the documentation is not very good but it looks sufficient for what we want to do with it and it is completed by the other API docs).
Here
it is said that: There is currently a limit of 300 templates and 300 versions that may be created per account.
To run example.js first run npm install in its directory and
complete the values of the first 4 variables declared at the beginning of the
script.
Other relevant links: NodeJS API docs, How to migrate from Mandrill, API docs.