Ones that I've heard of in conversation with customers:

   * IdentityServer (.NET)
   * The ORY suite (hydra, etc)
   * Gluu

I'd say Keycloak is the most common one I hear.

There are about 15 FOSS packages listed here, but you'll need to weed through what is an auth server and what is not: https://en.wikipedia.org/wiki/List_of_single_sign-on_impleme...

There is ForgeRock and WSO2 which we have used in the past. Perhaps they have moved on but would not recommend vs Keycloak.