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 47,787 websites that are phpBB Customers. We know of 11,666 live websites using phpBB and an additional 36,121 sites that used phpBB historically and 4,620 websites in the United States.
Get a list of 47,787 websites using phpBB which includes location information, hosting data, contact details, 16,862 currently live websites which includes 5,196 domains that redirect to sites in the list. 36,121 sites that used this technology previously and 4,620 websites in the United States currently using phpBB.