Elvanto

This plugin adds an Elvanto login widget to your WordPress powered site. 
When installed, your users will be able to log directly into their Elvanto account from your site.

Elvanto Login Widget

Quickly and easily let your users login to Elvanto from your WordPress powered site.

Bonny85

LB Users Counter is a plugin for WordPress. Allows you to add a widget to a sidebar, where you can view some attributes with respect to the number of users browsing the website (online, offline and online user name).
API Reference
User Class
This class contains the static methods for get info of users login
<ul>
<li>
User::getUserOff() This function get number of users offline.
</li>
<li>
User::getUserOn() This function get number of users online.
</li>
<li>
User::getNameUserOn() This function get the names of users online.
</li>
</ul>
<a href="http://www.lucabonaldo.it/lb-users-counter/" rel="nofollow ugc">Documentation in Italian</a>

LB Users Counter

Allows you to add a widget to a sidebar, where you can view some attributes with respect to the number of users browsing the website.

Elvanto Login Widget vs LB Users Counter