Freenode

freenode, formerly Open Projects Network or OPN, is a somewhat large network primarily devoted to the free software and open source community.

History
The network was originally founded in 1995 by the late Rob "lilo" Levin after moving from EFnet's #LinPeople. By 1998, it was renamed to Open Projects Network, and had about 200 users and 20 channels. By 2002, the Peer-Directed Projects Center, or the PDPC, was founded as an umbrella project for OPN and OPN was renamed freenode.

Today
Freenode is currently the largest FOSS network with roughly 40,000 users and over 10,000 channels. Of these channels, #ubuntu is the largest with over 1000 users, followed closely by #gentoo with over 900.