HexChat - HexChat is a fork of XChat with bug fixes and new features.
Empathy - Apps/Empathy - GNOME Wiki!
irssi - Irssi is a terminal based IRC client for UNIX systems.
Pidgin - Pidgin is an easy to use and free chat client used by millions. Connect to AIM, MSN, Yahoo, and more chat networks all at once.
mIRC - mIRC: Internet Relay Chat client
Trillian - Trillian is a decentralized and federated instant messaging platform that lets your whole company send private and group messages, keep tabs on what co-workers are doing, share files, and much more.