Since today, Google+ is offering a Sign-In service, just like Twitter, Facebook and Google are offering already. Yep, the last Google in the previous sentence is not a mistake. For years Google has been offering their platform as an authentication platform using a Google account for authentication via OAuth or OpenID.
Now Google is letting it sound like they are offering something new, and in fact, this could be true. I see no integration with the old authorized apps that they are offering for years. Over there I have a list of about 30 apps/sited that I have authorized, about 15 with the permission “Sign in using your Google account” and the rest more permission like accessing my calendar or mail. https://plus.google.com/apps is not showing any of that. I have not yet dug into the mechanism they are using, but I bet it is stock OAuth and thus this is not new and all the hype is BS.
The one example I tried is shown in the image and it using the old authentication method, not Google+. That makes me wonder how many of them are using Google+ already. Is this another try of Google to push a failing platform?