mIRC - mIRC: Internet Relay Chat client
HexChat - HexChat is a fork of XChat with bug fixes and new features.
irssi - Irssi is a terminal based IRC client for UNIX systems.
Kiwi IRC - A hand-crafted IRC client that you can enjoy. Designed to be used easily and freely.
Adium - Adium is a free instant messaging application for Mac OS X that can connect to AIM, MSN, Jabber, Yahoo, and more.
WeeChat - WeeChat is a fast, light and extensible chat client.