bbPress.org

I did this at http://aberean.org and http://forum.aberean.org

I used the WP Plugin (page links to) and then added a new page named forum to my WP site with a redirect (Page links to) to my forum.

With the forum I had to manually add links in the footer (or header) to link back. It was pretty easy and I think it looks ok. I have not themed the forum to mimic WP as of yet, to much else to do first!