Primary Link - http://www.Phpbb.com
Templates - available ( free also )
Plugins - available ( free also )
I must say thanks to phpbb developer for creating such a excellent framework and distributing it free. Very good framework and if you are going to start your forum site please just go for phpbb framework. phpbb has very good looks and good functionality. Installation is very easy and navigation is also very easy. Admin panel and site home page both have excellent graphics and easy navigation.
This is admin panel of phpBB3 , excellent graphics, large number of options are specialty of this framework.
Easy navigation is the key feature for using phpbb framework.
This is phpbb3 site's home page using phpbb3 as forum site is very easy and even if you have not installed any framework previously .You can use it.
Score - 10 / 10
Note - Please try at least once.