If you have multiple WordPress installations using the same database, this plugin makes it possible for one 
installation to run off the user table of another installation. Main user table selection is completely configurable 
through the options panel, and permissions of the users can be different from blog to blog.
When you switch the user table from one blog to another, all level 10 users (administrators) of the blog you are 
switching to are also made level 10 users on the blog in which you installed the plugin. Only a level 10 user can 
make the switch.
After you switch, you should visit the Users section of the Admin panel to setup permissions for all other users. 
They will be listed as “No role for this blog” in WordPress 2.3, or with an empty Role column under “All Users” in WordPress 2.5/2.6.
Note that ideally the blog you are switching should not have more than the default admin user. Content created by all other users 
will automatically be assigned to more or less random users from the main blog when you switch (based on <code>user_id</code>). Future versions 
of the plugin may support migrating existing users and content.

Shared Users

Have your blog run off the user table of another blog, in the same database.

st2i

WP OpenID Selector is the adaptation of openid-selector (http://code.google.com/p/openid-selector/) to wordpress.
Openid plugin (https://wordpress.org/extend/plugins/openid/) is needed to use wp-openid-selector.
Users who want’s to authenticate (register) on a wordpress blog with openid system, only need to click on the their logo’s provider, instead of writing it’s url, to be automaticaly redirected to their openid server and proceed to the authentication.
You can see the plugin in use on our web site (http://www.st2i.com) or on the stackoverflow web site.

WP OpenId Selector

Adaptation of OpenId Selector for wordpress

Shared Users vs WP OpenId Selector