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 48,026 websites that are phpBB Customers. We know of 11,304 live websites using phpBB and an additional 36,722 sites that used phpBB historically and 4,567 websites in the United States.
Get a list of 48,026 websites using phpBB which includes location information, hosting data, contact details, 17,380 currently live websites which includes 6,076 domains that redirect to sites in the list. 36,722 sites that used this technology previously and 4,567 websites in the United States currently using phpBB.