I'm not sure because i didn't ever use one, but look at this -> phpBB, it has a lot of features, it's PHP based, and it's free - I guess it's good too.
Anyway, you can take a look in this website and get a better idea.
I was also in search for a good forum software and thank God I came across to this page..Smoovo I think that website was fine.I had gathered data from it, It was useful...