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 47,442 websites that are phpBB Customers. We know of 8,140 live websites using phpBB and an additional 39,302 sites that used phpBB historically and 3,230 websites in the United States.
Get a list of 47,442 websites using phpBB which includes location information, hosting data, contact details, 8,140 currently live websites and an additional 4,426 domains that redirect to sites in this list. 39,302 sites that used this technology previously and 3,230 websites in the United States currently using phpBB.