Also noting that these days IBM is the main force behind Keycloak (through RedHat acquisition), so the saying fits even better.

If you are looking for an open source solution, it is definitely the 800 lb gorilla.

If you are looking for a commercial solution, I would say that the top competitors are Okta, Auth0, Azure AD, and Firebase (though the latter, from what I've heard, doesn't play nicely with standards).

And now that Okta and Auth0 are the same, the big two options are Okta/Auth0 and Azure AD. I don't know how much Firebase gets used at scale but the price "scales" so well that I think most people switch off of it after the MVP validation phase. Just conjecture though, no numbers to back that up -- an enterprise client is probably not going to use your product because it has Firebase login integration.

> And now that Okta and Auth0 are the same, the big two options are Okta/Auth0 and Azure AD.

I mean, it's yet to be determined how Okta and Auth0 are going to integrate. I have read that Okta has three sets of APIs, maybe Auth0 will be a fourth?

> an enterprise client is probably not going to use your product because it has Firebase login integration.

LOL. It might even be a negative signal.

But I hear Firebase is thinking about supporting standard OAuth, which would be great.

I wouldn't say they're the same thing except from a brand name perspective. Okta is barely app-focused; much more enterprise SSO. Auth0 is extremely app-focused, although they can do some enterprise SSO as well.

Okta bought Stormpath (and killed it) and now Auth0 because of this difference, and because there's a finite total addressable market for employees, but an almost infinite total addressable market for app users.