KFAN Rube Chat
A very active community for a KFAN-AM, a popular sports radio station in Minneapolis/St. Paul, MN.
Debian
Official Support Forum for Debian, the kernel-based open source operating system. phpBB provides a solid platform for support and discussion.
CitiesXL 2011
Developed by Focus Home Interactive, Cities XL 2011 is a city simulation game. Their community forums are powered by phpBB.
NationStates
Inspired by Max Berry's book, Jennifer Government, NationStates is one of the most popular online simulation games. The forum boasts over 5 million posts and 200,000 users.
xkcd
Discussion board for the massively popular xkcd web comic. Covering everything from Maths and Science to Logic Puzzles and Forum Games. Over 100k members and 2.4 Million posts.