joch

If you need to make your WordPress site private, this plugin makes it possible for 
your users having to log in before being able to view any content on the website.

WP Require Auth

This plugin makes it mandatory to be logged in before viewing any content.

Automattic

Joey Kudish

Michael Adams (mdawaffe)

Em Shreve

Beau Lebens

See the <a href="http://dentedreality.com.au/projects/wp-keyring/" rel="nofollow ugc">Keyring Developer’s Guide</a> for more details.
Keyring provides a very hookable, completely customizable framework for connecting your WordPress to an external service. It takes care of all the heavy lifting when making authenticated requests, so all you need to do is implement cool features and not worry about these tricky bits.
Out of the box, Keyring currently comes with base Service definitions for:
<ul>
<li>HTTP Basic,</li>
<li>OAuth1, and</li>
<li>OAuth2.</li>
</ul>
And includes ready-to-use definitions for:
<ul>
<li><a href="https://500px.com/" rel="nofollow ugc">500px</a></li>
<li><a href="https://delicious.com/" rel="nofollow ugc">Delicious</a></li>
<li><a href="https://eventbrite.com/" rel="nofollow ugc">Eventbrite</a></li>
<li><a href="https://facebook.com/" rel="nofollow ugc">Facebook</a></li>
<li><a href="https://fitbit.com/" rel="nofollow ugc">Fitbit</a></li>
<li><a href="https://flickr.com/" rel="nofollow ugc">Flickr</a></li>
<li><a href="https://foursquare.com/" rel="nofollow ugc">Foursquare</a></li>
<li><a href="https://www.google.com/analytics/" rel="nofollow ugc">Google Analytics</a></li>
<li><a href="https://www.google.com/contacts/" rel="nofollow ugc">Google Contacts</a></li>
<li><a href="https://www.google.com/mail/" rel="nofollow ugc">Google Mail</a></li>
<li><a href="https://instagram.com/" rel="nofollow ugc">Instagram</a></li>
<li><a href="https://instapaper.com/" rel="nofollow ugc">Instapaper</a></li>
<li><a href="https://jetpack.com/" rel="nofollow ugc">Jetpack</a>/<a href="https://wordpress.com/" rel="nofollow ugc">WordPress.com</a></li>
<li><a href="https://linkedin.com/" rel="nofollow ugc">LinkedIn</a></li>
<li><a href="https://moves-app.com/" rel="nofollow ugc">Moves</a></li>
<li><a href="https://nest.com/" rel="nofollow ugc">Nest</a></li>
<li><a href="https://pinterest.com/" rel="nofollow ugc">Pinterest</a></li>
<li><a href="https://runkeeper.com/" rel="nofollow ugc">RunKeeper</a></li>
<li><a href="https://strava.com/" rel="nofollow ugc">Strava</a></li>
<li><a href="https://tripit.com/" rel="nofollow ugc">TripIt</a></li>
<li><a href="https://tumblr.com/" rel="nofollow ugc">Tumblr</a></li>
<li><a href="https://twitter.com/" rel="nofollow ugc">Twitter</a></li>
<li><a href="https://yahoo.com/" rel="nofollow ugc">Yahoo! Updates</a></li>
<li><a href="https://youtube.com/" rel="nofollow ugc">YouTube</a></li>
</ul>
You can very easily write your own Service definitions and then use all the power of Keyring to hook into that authentication flow. See the <a href="http://dentedreality.com.au/projects/wp-keyring/" rel="nofollow ugc">Keyring Developer’s Guide</a> for more details.
Contributions are welcome via <a href="https://github.com/beaulebens/keyring" rel="nofollow ugc">Github pull request</a>.

Keyring

An authentication framework that handles authorization/communication with most popular web services.

WP Require Auth vs Keyring