Thats a random question for here. As for free forums go I don't think there are any without adds. Invision free can have no adds if you pay for it not to. You might want to look into .Mac groups. That might help but I don't think that it is the same as the Invision Free boards.
Phpbb works well, I've used it for years. It's had some significant security holes, though, so make sure you install the latest stable release and keep it updated.