I am running 4 forums and all of these forums are built on open source forum scripts like phpBB, SMF, and MyBB. I am even using WordPress Forum PLugin for one of my forums. The primary reason for using these scripts is I do not have to spend money to run my forums. I already had a domain and hosting, so my forum cost me zero dollars. One of my forums makes some money, which I can use for content building and marketing. If I had used a premium forum script, I would have ended up spending a lot of money just on the forum scripts.