BBPress is forum software written by the makers of WordPress. It can share user registration databases with wordpress, supports rss feeds and can be nice and lean. I’ve used it for a couple sites and will use this page to keep track of the official site, support forums, themes and extensions.
bbpress.org – Official bbpress.org site.
bbpress forums – Official forums at bbpress.org
bbpress plugin directory – Official bbpress plugin directory
bbshowcase forums – bbshowcase forums. This is a great source for ideas and help with bbpress and also a good resource for extensions or plugins for bbpress.