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.

curlybracket

This plugin will allow you to automatically rehash the passwords of users you have beforehand imported from a Prestashop database to your WordPress database.
This is useful if you are migrating a website from Prestashop to WordPress.
It allows users to authenticate via their email address, like on a Prestashop installation.
Upon login, the plugin will check the password entered by the user, against the value stored in the database. Using the COOKIE_KEY of the former Prestashop installation, it’ll verify if the password is correct and, only in that case, it’ll rehash the password using WordPress’ default hashing algorithm and insert the new password to the database. Then it’ll try logging in.
In every other case, login will function normally, so if there is an error, it’ll be returned.
In a while, every former Prestashop user you have imported to your WP DB will have their passwords securely rehashed.
First tested successfully with Prestashop 1.3.5 and WordPress 3.6. 
Lately tested successfully with Prestashop 1.5.6.1 and WordPress 3.9.

Prestashop User Compatibility

This plugin will automatically rehash the passwords of users you have beforehand imported from a Prestashop database to your WP database.

Shared Users vs Prestashop User Compatibility