My web site is running on joomla 3.0 and the choice of free forum is small enough, so I could try everything available. I can say that my first choice is Chronoforums for three reasons :
1) It has all the necessary features for a forum (moderator, email notification for new post and new topics, etc.) with out unnecessary gadgets !
2) It is very simple to understand and to install and to get it running
3) At last but not least if you do needs help, you can contact Max, which is very reactive and deadly efficient ! On demand, Max added some special features I needed and gave me some help and advise without delay and totally free !
So if you are looking for a simple but effective Forum and you wish to spend time answering messages instead of trying to understand how your forum is working, then stop searching here it is.