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.

Jon Brown

Adds a metabox to the menu admin page listing users so they can be more easily added as custom links to the WordPress Custom Menus.

User Links for WP Menus

Adds a metabox to the menu admin page listing users so they can be more easily added as custom links to the WordPress Custom Menus

Duplicate Comment Authors vs User Links for WP Menus