A WordPress plugin that allows users to login or register by authenticating with an existing Google, LinkedIn, Github, or other account via OAuth 2.0. Easily drops into new or existing sites, integrates with existing users.
Visit the WordPress Plugin Directory for more.
This plugin shares a background with wp-oauth.
There are many other such plugins available for Wordpress. The main difference here is there is no external service / SaaS component: it entirely operates within your own site.
This is released under GPLv2 and is free (as in speech and also as in beer). There is no 'premium' version.