We've previously used the Discussion Forums http://www.concrete5.org/marketplace/addons/discussion... addon from Portland Labs themselves and it works really well. You can see our implementation here, http://digdeepcoaching.com/forum... although it's not been used very much yet (it was added quite recently).
There is also Basic Forum http://www.concrete5.org/marketplace/addons/basic-forum... which has few features except it's price (free!).
One thing to note though is that there isn't currently any forum addons for version 5.7.
Hope that helps.