Question: In my host, I currently have installed 2 wordpress applications, 1 phpBB forum and one MediaWiki.

Is there a way to merge the login so that all applications share the same credentials?

For instance, I want to register only in my phpBB and then I want to access all other applications with the given username and password.

Even if you don't know a unified way, what other login integration do you know of? Pros and cons of each?

Answer: 
I don't know how to share the session cookies, but you can easily share the same login. 
i.e. People will need to log separately into both sites, but will be able to use the same username and password. 

In the mediawiki file "LocalSettings.PHP", you can tell it to use a different (wordpress) database for authentication: 

e.g. 

```
require_once('includes/AuthPlugin.php');
require_once('extensions/AuthPress.php');

$wgAuth = new AuthPress();
$wgAuth->setAuthPressTablePrefix('evo_');
# Only include the following if you aren't using the same db as  MediaWiki
$wgAuth->setAuthPressDBServer ('localhost');
$wgAuth->setAuthPressDBName('yourWordPressDB');
$wgAuth->setAuthPressUser('mySQL user for same');
$wgAuth->setAuthPressPassword('The password');

```

See <http://bbpress.org/forums/topic/mediawiki-bbpress-and-wordpress-integration>
If you're integrating a bunch of different apps, and you really just want a bridge, I've had good success with the bridge from [Single-Signon.com](http://www.single-signon.com). You can see there supported apps here:
<http://www.single-signon.com/en/applications.html>

I've also used a MediaWiki extension for phpBB integration:
<http://www.mediawiki.org/wiki/Extension:PHPBB/Users_Integration>