If you have a wordpress based site simple press forums from stuff at yellowfish is awesome. I have it installed on most of my sites even if I have had trouble attracting members, simply because it gives me another avenue of built in
SEO. Feeds and a place to create user profiles with well placed links are just two options. It also has a variety of options when it comes to pretty permalinks and tags something you are hard pressed to do well in other forum software.
They just released the final version of 4.0 and it works really well within any well made wordpress theme. Depending on your size requirements you might have to make a template file for your forum page to adjust your standard blog sidebar on the forum page, but that is very simple to do.