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.

Ivan

The Google Authenticator plugin for WordPress gives you two-factor authentication using the Google Authenticator app for Android/iPhone/Blackberry.
If you are security aware, you may already have the Google Authenticator app installed on your smartphone, using it for two-factor authentication on Gmail/Dropbox/Lastpass/Amazon etc.
The two-factor authentication requirement can be enabled on a per-user basis. You could enable it for your administrator account, but log in as usual with less privileged accounts.
If You need to maintain your blog using an Android/iPhone app, or any other software using the XMLRPC interface, you can enable the App password feature in this plugin, 
but please note that enabling the App password feature will make your blog less secure.
<h3>Credits</h3>
Thanks to:
<a href="https://profiles.wordpress.org/evinak/" rel="nofollow ugc">Oleksiy</a> for a bugfix in multisite.
<a href="https://github.com/pancek" rel="nofollow ugc">Paweł Nowacki</a> for the Polish translation
<a href="https://github.com/FabioZumbi12" rel="nofollow ugc">Fabio Zumbi</a> for the Portuguese translation
<a href="https://www.guidoschalkx.com/" rel="nofollow ugc">Guido Schalkx</a> for the Dutch translation.
<a href="https://www.paypal.com/cgi-bin/webscr?cmd=_donations&business=henrik%40schack%2edk&lc=US&item_name=Google%20Authenticator&item_number=Google%20Authenticator&no_shipping=0&no_note=1&tax=0&bn=PP%2dDonationsBF&charset=UTF%2d8" rel="nofollow ugc">Henrik.Schack</a> for writing/maintaining versions 0.20 through 0.48
<a href="http://tobias.baethge.com/" rel="nofollow ugc">Tobias Bäthge</a> for his code rewrite and German translation.
<a href="http://blog.pcode.nl/" rel="nofollow ugc">Pascal de Bruijn</a> for his “relaxed mode” idea.
<a href="http://technobabbl.es/" rel="nofollow ugc">Daniel Werl</a> for his usability tips.
<a href="http://dd32.id.au/" rel="nofollow ugc">Dion Hulse</a> for his bugfixes.
<a href="https://profiles.wordpress.org/users/aldolat/" rel="nofollow ugc">Aldo Latino</a> for his Italian translation.
<a href="http://www.kaijia.me/" rel="nofollow ugc">Kaijia Feng</a> for his Simplified Chinese translation.
<a href="http://www.buayacorp.com/" rel="nofollow ugc">Alex Concha</a> for his security tips.
<a href="http://jetienne.com/" rel="nofollow ugc">Jerome Etienne</a> for his jquery-qrcode plugin.
<a href="http://orizhial.com/" rel="nofollow ugc">Sébastien Prunier</a> for his Spanish and French translation.

Google Authenticator

Personally I feel 2FA should be a part of WordPress core. But since it isn't, plugins like this one are a must-have to secure your sites.

The configuration interface is clearly layed out and the login page fits in perfectly with the default WP login page styling.

This plugin is light-weight, unobtrusive, free and adds that extra layer of security. Should your admin credentials ever be compromised, 2FA will keep your site secure.

Must-have security plugin

Awesome plugin, which is almost always in first group of plugin I've install on new website. Thank you for create and maintain it.

One on few my first-to-install plugin

Google Authenticator for your WordPress blog.

Shared Users vs Google Authenticator