phpBB is definitely one of the most popular free forum software on the net, and is often talked about next to MyBB and SMF. It's also older than the two, making it's debut in December of 2000 with both SMF and MyBB being released in 2002. I think it actually might be the oldest forum software that's still being actively being updated and is still widely used. I feel that it's not the best software for beginner forum owners, since the permissions are a pain to set up correctly. I also feel that it's a bit outdated compared to MyBB and SMF. Though I kind of have a soft spot for it, since one of the first forums I ever joined was a Teen Titans forum that used phpBB 2.