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bbPress(.org) is free and by the creators of WordPress and BuddyPress.
Let me know if you need help

Cheers,
Aron

I second bbPress, and buddypress.

You may also want to check out xenforo. It's paid, but it's pretty sleek software.

Hope that helps.

I use bbpress on a few websites and some wp templates require you to have it so I kind of had no choise but to use it the most

Nothing better than bbpress.