If you're looking for a good free PHP based forum script I highly recommend MyBB. I used to use SMF but once I switched over to MyBB I haven't looked back. The only cons of MyBB are that there aren't as many themes or plugins but the ones that do exist are pretty good. In addition, the support forum is VERY useful and a lot of the members on there will really go out of there way to help you which is great.
I'm concerned about security, too. Sure open-source is great, but without a community like that behind Firefox, I don't know how secure these smaller forums actually are. All open-source forums should merge into one to get a bigger developer community.
Until then, splurge for vBulletin if you can. It's much easier to admin from the CP.
Last edited by webfreak08 : 09-07-2009 at 06:32 PM.