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 47,077 websites that are phpBB Customers. We know of 9,615 live websites using phpBB and an additional 37,462 sites that used phpBB historically and 3,839 websites in the United States.
Get a list of 47,077 websites using phpBB which includes location information, hosting data, contact details, 9,615 currently live websites and an additional 4,638 domains that redirect to sites in this list. 37,462 sites that used this technology previously and 3,839 websites in the United States currently using phpBB.