I was just thinking this same thing.
I believe all the tools are available to make one - so maybe even a good tutorital would be great. The guys at Interakt had a "How to build a Forum" tutorial build right into their "Kollections" (AKA: ADDT) manual, along with other great how-to's. These came in really handy as the building blocks for a custom approach.
WA has their recipes which are quite similar. Maybe a good "forum" recipe?