I work for FusionAuth which has a free community edition[0], so I'm a bit biased. You can read some of our community stories talking about what folks have built on top of it[1].

Other alternatives I've heard mentioned for self-hosting include:

* Ory

* Platform specific OSS (Devise for Rails, Passport/NextAuth for javascript, Spring Security for Java, etc)

* IdentityServer (may have to pay something now, not sure)

* Keycloak (as other comments have mentioned)

* Supertokens

0: https://fusionauth.io/download

1: https://fusionauth.io/blog/tag/community-story/