keycloak
hydra/kratos
fusionauth (not FOSS; not very good for a public internet application - probably ok for internal apps)
gluu
hydra/kratos
fusionauth (not FOSS; not very good for a public internet application - probably ok for internal apps)
gluu
Just thought I would mention that FusionAuth is powering a bunch of huge public internet applications with millions of MAU. Our largest customers have 10s of millions of total users as well.
We built it to scale to any size so don't worry about size.
Plus, it's free! :)