We have postponed for too long to add a user's discussion forum to Sampa. We are looking for a great forum solution for us to use. Do you know any? Here is what I describe as the perfect solution:
It must allow users to do all the basic tasks of forums;
It must have an administration panel, that allows us to control posts, moderate, create/delete forums, etc.
It must be secure and proven (not XSS vulnerabilities, spam prone, etc.)
It must allow us to override the authentication system, this is, Sampa users already have their account with Sampa. I don't want them to have to create an account on the Forum.
We must be able to "skin" it to fit the Sampa brand;
It doesn't matter if it is hosted or server-install;
It doesn't matter if it is Open Source or Proprietary;
It doesn't matter if it is free, shareware or cost real dollars;
Two key resources are: http://www.forum-software.org/ http://www.forummatrix.org/
The most-used candidates are probably vBulletin ($) and phpBB (F/OSS). vBulletin has a very active developer community producing extensions and has a very good reputation; since you're willing to pay, it may be a no-brainer choice for you.
phpBB is the most-used Open Source choice, but has had security issues, and is going through a major rewrite; I wasn't willing to be an early adopter of the new version.
Next bunch of candidates is punBB (simple), Phorum (ugly but robust), FUDforum, SMF/Simple Machines, and Vanilla. I hesitantly settled on FUDforum and hacked together my own custom user authentication via my Drupal front-end--was pretty easy. Site has seen limited use, so I can't give a real judgment.