I'd go with phpBB; it's free, customizable, open-source, widely used, usually fairly easy to install, and feature-rich. You'll get a lot of support and will find it's basically just a compact version of the powerful vBulletin software used here. The other main alternative is
XMB, which I have barely any experience with, but it's free and I've heard it's good. If you're not up for this kind of forum, and just want a simple message board, try
Phorum.