bbPress.org

How about a forum plugin for WordPress?

http://www.stuff.yellowswordfish.com/simplepress-forum/

http://www.xdweb.net/wordpress/xdforum2/ (old and probably dead)

http://www.fahlstad.se/wp-plugins/wp-forum/ (had some security issues, you should read to see if they’ve been taken care of)

I’ve never used any of them, I just know they exist for putting a forum on a WordPress page.