Ben May

This plugin prevents imposters from leaving comments on your website with either the same name or email address as a registered user on your site.
This prevents users on the blog from being impersonated.

Duplicate Comment Authors

Prevents people posting comments using the same name as a registered users display name.

isevcik

Plugin add new widget User Box Widget. Widget shows registered user avatars in a box in similar way 
like facebook like box does. 
You can tune widget with these options:
<ul>
<li>define title of widget box</li>
<li>set number of displayed avatars</li>
<li>choose shape of avatars: circle or rounded rectangle</li>
</ul>
If you have any problems or question contact me on http://www.ivansevcik.cz/user-box

User Box

Very easy to use widget for displaying box with user avatars.



Simple Users Box

Plugin provides a widget for displaying avatars of registered users.

Duplicate Comment Authors vs User Box