Freenode has it's roots on EFnet channel #LinPeople which was originally a Linux help channel with just 4 people, forming their own network in 1995 as irc.linpeople.org. In 1998 it changed to Open Projects Net (OPN) with roughly 200 users and under 20 channels. In 2008 Freenode peaked at 70,000 users and roughly 40,000 channels.
Freenode is for FOSS communities, non-profit organizations and related communities/organizations. Other kind of channels are forbidden.
Unlike most other networks Freenode is centrally managed, nodes are donated rather than linked to the network. All IRCOps has same privs to all servers.
Recommended server is chat.freenode.net which load balances across all the network nodes. You may however connect directly to a specified server as well.