Today Google announced its competitor to Facebook Connect, Google+ Sign In. Basically, you can use your Google credentials to sign into any site that has this feature enabled.
Sounds like the key difference is you can choose whether your friends see your interactions with a site or application. Apparently Facebook spams this to your wall (I was not aware of this, how annoying!), but G+ allows you to choose whether everyone or no one gets the information.
Given Googles ubiquity, this would seem very likely to be quite popular. In fact, I need to come up with some sort of app so I can try it out! lol.