A serverless OAuth "server" to implement the OAuth2 protocol in a serverless environment.
For tiny applications with a tiny user bases, free is better than a few hundred annually for the electricity to run a server at home. This is prohibitively expensive at enterprise levels of users, so get an EC2 instance (or several, to properly load balance &c.) instead.
It's a basic implementation exercise. It's nice to have it available for Proof of Concept applications.