One free forum host that I stumbled across recently is called freeforums.org and so far, I am incredibly impressed! Although the forum is ad supported, they let you you do revenue sharing so that you can make a little money for yourself.

The biggest drawback they have which I noticed is that they do not support PHPBB3 yet, but it's in the works.

Customer service is speedy and helpful however, so that's a big plus for me.