Since its creation in 2000, phpBB has become the most widely used Open Source forum solution.
http://www.phpbb.com/
Content Management System · Forum Software
Get access to data on 47,493 websites that are phpBB Customers. We know of 10,618 live websites using phpBB and an additional 36,875 sites that used phpBB historically and 4,198 websites in the United States.
Get a list of 47,493 websites using phpBB which includes location information, hosting data, contact details, 10,618 currently live websites and an additional 4,991 domains that redirect to sites in this list. 36,875 sites that used this technology previously and 4,198 websites in the United States currently using phpBB.