An IRC client. Preferably used in conjuection with screen

screen -S irc irssi will give you a named screen session called <somePid>.irc, so later, when you have detached your screen (C^a d), logged off, done other things, come back, and want to resume your session, you could just do screen -ls and get a listing of all your currently running screens, followed by screen -d -r <somePid>.irc, and you are back where you left off.

Neat huh?