Hanko is a self-hostable authentication service. Alternatively, the managed cloud service can be used instead of self-hosting.
Hanko offers 10000 monthly active users for free. One issue however, is that inactive projects will be paused after 7 days of inactivity.
A similar alternative is Supertokens, which is also self-hostable. An alternative for a non-open-source managed auth service is Clerk