If you’re still writing your own authentication for your websites, you may want to get with this program. Have a look at the bottom right of this page:
Yep, almost everybody has one of those accounts these days, and more and more of those users are getting tired of endlessly multiplying username/password combinations.
I realize that Microsoft tried to do some Passport service in the past, but you can base your website’s auth on any site or combination of sites, even if they don’t have an API. As Tony Stubblebine has put it: “A login form is an API.”