IRC

IRC allows you to talk to people all over the world in channels that can have 1 or 2 thousand people.

Clients
irssi

erc

XChat

Hexchat

Servers
UnrealIRCd

Networks
DALnet

EFnet

Freenode

IRCnet

PTNet

QuakeNet

Rizon

SwiftIRC

Undernet

Ustream

WebChat