Ismail

Does the same functionality as BuddyPress, emailing users who get mentioned in a bbPress topic or reply. The aim of this plugin is when to enable that feature when no BuddyPress is there. 
To do: – warn user about bbPress/BuddyPress presence

bbPress Mentions Email Notifications

Adds the @mentions to bbpress - really useful

Great plugin

Email bbPress topic/reply mentioned users, set custom email and some basic settings

BuddyDev

Brajesh Singh

WordPress Username availability checker for WordPress & BuddyPress checks for the username availability on new user registration screens. 
It assist users & site admins by notifying them using ajax whether the username they entered is available or not.
<h4>Features</h4>
<ul>
<li>Checks for username availability in WordPress Admin New User screen</li>
<li>Checks for username availability on WordPress standard registration page</li>
<li>Checks for availability on BuddyPress registration page.</li>
</ul>
For screenshots & more details, please visit <a href="http://buddydev.com/plugins/bpdev-username-availability-checker/" title="Plugin page" rel="nofollow ugc">BuddyDev Username Availability Checker plugin page</a>
Free & paid supports are available via <a href="http://buddydev.com/support/forums/" title="BuddyDev support forums" rel="nofollow ugc">BuddyDev Support Forum</a>. Please Use BuddyDev support forums for your support requests. We may not be able to keep an eye over WordPress plugin support forum.

WordPress Username Availability Checker

WordPress Username availability checker checks if a username is available or not.

bbPress Mentions Email Notifications vs WordPress Username Availability Checker