KDE is mainly known for producing the KDE Desktop Environment (used in Kubuntu) for Linux.
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.
Discussion forum for the massively popular American animated sitcom. Powered by phpBB.
Pixelmator is a popular commercial image editing software for Mac OS X users.
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.
Hugely popular Mozilla community providing support for Firefox, Thunderbird, Seamonkey and other well known Mozilla products. Over 350k members and 4 Million posts.