OpenCart is popular, powerful, and easy to use shopping cart software. Their healthy community forums are powered by phpBB.
OpenOffice.org offers a free and fully-featured office suite which meets the needs of the common consumer all the way up to large enterprises.
Gentoo Linux is an open source operating system built on top of the Linux kernel and based on the Portage package management system.
KDE is mainly known for producing the KDE Desktop Environment (used in Kubuntu) for Linux.
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.
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.