There is one very big concern about free forum hosting:
Most do not allow you to back it up yourself and have a copy of it...meaning if for any reason they decide to delete it, all the posts are gone forever. Or if you decide to move your forum off their site, you can't take your database with you.
And you can't tweak it any farther than editing the html/css in the template to alter it's look, and from what I have seen at Nabble, they don't even allow you to do that.
If you are looking for free Invision Power Board hosting, Forumer.com
might be an option to consider. But like I said...they own your database and you will never get a copy of it. But at least they let you personalize the look. (they also slap google ads on it)
This was my forum
hosted by Forumer for my chatroom before it's demise. (I never even got to finish tweaking the template
And this is what the basic untweaked forum