onigoetz

This plugin allows to use the password from a working phpBB installation to log into a WordPress blog. or on the other side to log into phpBB with the WordPress user.
The username for the administration must be the same on the two installations. 
Otherwise you will end having admins that have no privileges on the other part.
<h4>How it works</h4>
There is no actual “sync” of the users.
When the app is configured correctly. the app checks that the user who tries to log in is at least registered in one of the systems, if yes, and the password is correct : he’s logged on both systems and his account is created if missing. 
On password change it should also set the password on both platforms.
<h4>WP2BB</h4>
With WP2BB, you can now define that your posts are published in a forum, so that your community can add comments directly on them.
You can add the method “wp2bb()” on your template so that you have links to your forum to add comments and a replies count
There is also a widget to list the last replies in the forums
P.S. : WP2BB was created by Alfredo de Hoces (http://www.alfredodehoces.com/)
<h3>Uninstall</h3>
To uninstall, you have a few steps to make
Go the the Plugin options, revert the “auth mode” to “DB” and not “wpbb” 
If it doesn’t work go to the ACP in General -> Client communication -> Authentication
Then go to the wordpress control panel and disable the plugin.
The last step is then to go with FTP in the PHPBB folder and rename the file “common.php” to “common.wp.php” and the file “common-orig.php” to “common.php”.
Your configurations should be separated again

PHPBB Single Sign On

Authenticate in Wordpress and phpBB at the same time

skipstorm

This plugins adds a widget with the latest posts from your phpbb3 forum. 
To make it work you have to add a file on your forum directory wich is easily configurable to fetch the posts from a specific category or the whole forum. 
This plugins supports multiple istances so you can display topics from different forums or categories in multiple widgets.

Sk Latest Post Widget

This plugins adds a widget with the latest posts from your forum.

PHPBB Single Sign On vs Sk Latest Post Widget