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 47,279 websites that are phpBB Customers. We know of 9,147 live websites using phpBB and an additional 38,132 sites that used phpBB historically and 3,634 websites in the United States.
Get a list of 47,279 websites using phpBB which includes location information, hosting data, contact details, 9,147 currently live websites and an additional 4,756 domains that redirect to sites in this list. 38,132 sites that used this technology previously and 3,634 websites in the United States currently using phpBB.