GromBeestje

The OAuth plugin for WordPress allows you to provide authentication against any 
OAuth provider. This plugin is uses the OAuth library by Manuel Lemos. The 
plugin allows the admin to select any from the services supported 
out-of-the-box by the said library. Additionally it is possible to configure a 
service manually, which means any site supporting the OAuth protocol can 
be used.

BlaatLogin: OAuth

This plugin turns your WordPress website into an OAuth Consumer. It allowsallows your users to sign in with any OAuth provider.

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.

BlaatLogin: OAuth vs Keyring