There are tons of free forums. There are few that are as popular, stable or mature as phpBB. The other side of the coin is that it being the most popular also rings true for exposed vulnerabilities.
Many people go for lighter forums, since many do not need the extra resource usage that phpBB requires.
I'm using Viscacha on one of my sites right now, just to try something new. It's very stable and has got a lot of features, but for my purposes, the features are more useful than phpBB's.
thanks,
json