phpBB is completely free and easy to set up (5 minutes and you're running). It's BTW the forum software DAZ is using (although they use an old version).

That been said, BadKittehCo has a point: There are about as many forums as there are users in Poserland, and people only visit their fav 2-3 ones. You really need a strong reason for people to go there, and, most important, to come back again, and again, for months and years to come.