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: 
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>
I personally think that integration login systems is one of, if not the, hardest job when utilizing multiple prebuilt applications. As a fan of reuse and modularity, I find this disappointing. If anyone knows of any easy ways to handle this problem between random app X and random app Y, I would love to know.