Philip Hofstetter back in 2008 – Why is nobody using SSL client certificates?:

"Did you know that ever since the days of Netscape Navigator 3.0, there is a technology that allows you to

– securely sign on without using passwords,

– allow for non-annoying two-factor authentication,

– uniquely identify yourself to third-party websites without giving the second party any account information

All of this can be done using SSL client certificates."

(Sounds like a better Single Sign-On solution to me than BrowserID.)