Official Support Forum for Debian, the kernel-based open source operating system. phpBB provides a solid platform for support and discussion.
GameServers is a premier Game Hosting company, having collaborations with big names such as Infinity Ward, Treyarch and Epic Studios.
Professional online broker and trading community.
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.
KDE is mainly known for producing the KDE Desktop Environment (used in Kubuntu) for Linux.
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.