1. If you are thinking about vBulletin, it costs money anyway (and would cost you more than your hosting fees on a cheap host).
2. Planning on phpBB ? I still strongly suggest to get a cheap hosting account (there are many hosting companies that can host you for $5 / month). Try out for 1 month, and then continue paying if you like it.
Having experience with a lot of free hosts in the past, my suggestion would be to go for paid hosting if you are serious about setting up your forum.
Good Luck