irssi - Irssi is a terminal based IRC client for UNIX systems.
HexChat - HexChat is a fork of XChat with bug fixes and new features.
WeeChat - WeeChat is a fast, light and extensible chat client.
mIRC - mIRC: Internet Relay Chat client
KVIrc - KVIrc is a free portable IRC client based on the excellent Qt GUI toolkit.
Quassel IRC - Quassel IRC is a modern, cross-platform, distributed IRC client based on the Qt4 framework.