This does not require a plugin, this can be done on each wordpress installation via the wp-config.php file. Just add the following code, I have this set under define('WP_DEBUG', false); on my setup.
wp_users , wp_usermeta being the name of the user database you want to utilize, so change this to whatever yours may be. This is a free and easy solution. This however only appears to allow the login credentials between sites, meaning you would have to log into each site.