For those of you looking to add very simple forums to your own site, I've got a little forum package that I originally created as a proof of concept for another project.  It turned out to be useful on its own though.  You can see it at http://www.lazarusid.com/forum/forum.cgi, or you can get info on putting it on your own site at http://www.lazarusid.com/forum.shtml

It requires no registration and offers basic inline formatting as well as automatic http URL linking.  It doesn't allow embedded HTML, and doesn't require registration by your users.  This makes it ideal if you are looking for a low cost of entry solution that encourages participation.  Full source is included so that you can modify it to meet your needs.  There are currently no known security holes, and the feature set is limited enough that there aren't likely to be any.