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.

jystewart

Many people use wordpress to run blogs alongside their Ruby on Rails applications. And many 
Ruby on Rails applications use very similar login systems, derived from the restful_authentication 
or clearance plugins. This plugin provides hooks so that your wordpress blog can read your rails 
app’s database.yml file, connect to that database and authenticate the user against your rails app.
This plugin requires the syck PHP extension for parsing yaml. Instructions for installing syck are available at 
http://trac.symfony-project.org/wiki/InstallingSyck
<h3>To Do</h3>
<ul>
<li>Add automated tests</li>
<li>Solicit user feedback on further options</li>
</ul>

WP Rails Authenticate

Log users in against their accounts in a Ruby on Rails application.

Shared Users vs WP Rails Authenticate