OpenOffice.org
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.
Mozillazine
Hugely popular Mozilla community providing support for Firefox, Thunderbird, Seamonkey and other well known Mozilla products. Over 350k members and 4 Million posts.
Ubuntu Italy
It is the Italian Community for Ubuntu, a popular open source community operating system based upon the the Debian Linux Distribution.
Synology
Support for Synology NAS devices and ideas for future versions of the DiskStation software.
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.