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.

Functions:
<ul>
<li>Simple LogIn (one click)</li>
<li>Simple LogOut (one click)</li>
<li>Create user automatically</li>
<li>Update user’s profile with the informations that came from Identity Provider</li>
<li>Protect the user’s profile, that the user can’t modificate the informations that came from Identity Provider</li>
<li>Syncronize the LogIn and LogOut actions between the WordPress and the Identity Provider </li>
<li>If the (Apache) agent have a long update period you can choose what to do the plugin: go to login URL or LogIn.</li>
<li>SSL support</li>
<li>Rules: If a regular expression match to the $_SERVER variable’s element put the user to the group.</li>
<li>Plus plugin: Simple Group Manager – Manage the user’s groups and the group’s roles.</li>
<li>Admin page: Users / SSO Autentikáció</li>
</ul>
For more infromation please read this: http://kir-dev.sch.bme.hu/wp-opensso
If you don’t now how it’s work: https://redmine.kirdev.sch.bme.hu:8081/wiki/wordpress/OpenSSO_autentikacio (in Hungarian)

OpenSSO

Authenticate users using  Sun's Open Single Sign On service.

Shared Users vs OpenSSO