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.
Hugely popular Mozilla community providing support for Firefox, Thunderbird, Seamonkey and other well known Mozilla products. Over 350k members and 4 Million posts.
Password Manager, Auto Form Filler, Random Password Generator & Secure Digital Wallet App.
Official Support Forum for Debian, the kernel-based open source operating system. phpBB provides a solid platform for support and discussion.
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.