Google announced today that it is rolling out Google+ sign-in support for both apps and the web, giving users one-click login to third party services with their G+ account. Technically called "application sign-in", this new feature will replicate what many are used to with plain old Google sign-in, but now with enhanced features that come along with Google+. Similarly to the way sites and apps can now authenticate you with your Facebook or Twitter accounts, Google+ will now be an option when visiting sites that have integrated the service.
You get all of the perks of having site credentials tied to your Google Account, as well as the improved sharing functions that come along with Google+. When sharing from an app that has the service integrated, people that see the post on Google+ will be able to directly act on the item you shared -- for example sharing a radio program from TuneIn Radio will give a direct "listen" button right in the post, or sharing a listing on Open Table will give a button to view the restaurant's menu. This is very similar to the way Google already integrates app installs and music purchases into Google+ posts.
Developers that use Google+ sign-in on the web will also be able to offer users their Android apps directly from their own website, with a pop-up "install" screen that looks identical to the one found on the Play Store. About a dozen different sites are on board with this initial launch of the new application sign-in, with more on the way. It will be interesting to see how many apps take advantage of the new Google+ sign-in option and sharing improvements.
Source: Google Developers Blog