BuddyPress.org

If you’re just looking for an easy-to-integrate forum for WordPress, try Simple:Press.
If you want to integrate a standalone forum into WordPress, try out bbPress.

To find out if BuddyPress is for you, try out BuddyPress over on the test installation site – http://testbp.org.
Although BuddyPress does have some forum functionality, it is not a dedicated forum script!

Hope that helps somewhat.