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: 
when you integrate the system. Just remember 2 things:

1. Login to system  

Check username/password with both systems.
2. Change of Password  

Update the password on both systems.
I once did a phpBB/MediaWiki login integration from the phpBB end.

[Check it out](https://damnian.svn.sourceforge.net/svnroot/damnian/phpBB3/MediaWiki/).