Before Akiva bought it, it was written/sold by O'Riley. It is a commercial product that has been around for a long time. At one time it was (arguably) the most powerful web based discussion forum software. It was also one of the most expensive.
The product has broadened out to be more CMS than straight forum. It would be very interesting to see it reviewed alongside all the great software you have listed here.
It can be found at www.akiva.com.