phpBB

With a large and highly customisable set of key features coupled with a standards compliant interface, phpBB will grow with, and complement your website. With millions of installations worldwide, it is no wonder phpBB is one of the most popular ways to create and support a community.

Read more: http://www.smashingapps.com/2009/11/26/15-most-popular-free-software-i-am-thankful-for.html#ixzz2MCAiwuDq